Refactor: Move login out of models (#16199)

`models` does far too much. In particular it handles all `UserSignin`.

It shouldn't be responsible for calling LDAP, SMTP or PAM for signing in.

Therefore we should move this code out of `models`.

This code has to depend on `models` - therefore it belongs in `services`.

There is a package in `services` called `auth` and clearly this functionality belongs in there.

Plan:

- [x] Change `auth.Auth` to `auth.Method` - as they represent methods of authentication.
- [x] Move `models.UserSignIn` into `auth`
- [x] Move `models.ExternalUserLogin`
- [x] Move most of the `LoginVia*` methods to `auth` or subpackages
- [x] Move Resynchronize functionality to `auth`
  - Involved some restructuring of `models/ssh_key.go` to reduce the size of this massive file and simplify its files.
- [x] Move the rest of the LDAP functionality in to the ldap subpackage
- [x] Re-factor the login sources to express an interfaces `auth.Source`?
  - I've done this through some smaller interfaces Authenticator and Synchronizable - which would allow us to extend things in future
- [x] Now LDAP is out of models - need to think about modules/auth/ldap and I think all of that functionality might just be moveable
- [x] Similarly a lot Oauth2 functionality need not be in models too and should be moved to services/auth/source/oauth2
  - [x] modules/auth/oauth2/oauth2.go uses xorm... This is naughty - probably need to move this into models.
  - [x] models/oauth2.go - mostly should be in modules/auth/oauth2 or services/auth/source/oauth2 
- [x] More simplifications of login_source.go may need to be done
- Allow wiring in of notify registration -  *this can now easily be done - but I think we should do it in another PR*  - see #16178 
- More refactors...?
  - OpenID should probably become an auth Method but I think that can be left for another PR
  - Methods should also probably be cleaned up  - again another PR I think.
  - SSPI still needs more refactors.* Rename auth.Auth auth.Method
* Restructure ssh_key.go

- move functions from models/user.go that relate to ssh_key to ssh_key
- split ssh_key.go to try create clearer function domains for allow for
future refactors here.

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/auth/sspi_windows.go

@@ -15,6 +15,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/web/middleware"
+	"code.gitea.io/gitea/services/auth/source/sspi"
 
 	gouuid "github.com/google/uuid"
 	"github.com/quasoft/websspi"
@@ -32,7 +33,10 @@ var (
 	sspiAuth *websspi.Authenticator
 
 	// Ensure the struct implements the interface.
-	_ Auth = &SSPI{}
+	_ Method        = &SSPI{}
+	_ Named         = &SSPI{}
+	_ Initializable = &SSPI{}
+	_ Freeable      = &SSPI{}
 )
 
 // SSPI implements the SingleSignOn interface and authenticates requests
@@ -146,7 +150,7 @@ func (s *SSPI) Verify(req *http.Request, w http.ResponseWriter, store DataStore,
 }
 
 // getConfig retrieves the SSPI configuration from login sources
-func (s *SSPI) getConfig() (*models.SSPIConfig, error) {
+func (s *SSPI) getConfig() (*sspi.Source, error) {
 	sources, err := models.ActiveLoginSources(models.LoginSSPI)
 	if err != nil {
 		return nil, err
@@ -157,7 +161,7 @@ func (s *SSPI) getConfig() (*models.SSPIConfig, error) {
 	if len(sources) > 1 {
 		return nil, errors.New("more than one active login source of type SSPI found")
 	}
-	return sources[0].SSPI(), nil
+	return sources[0].Cfg.(*sspi.Source), nil
 }
 
 func (s *SSPI) shouldAuthenticate(req *http.Request) (shouldAuth bool) {
@@ -177,7 +181,7 @@ func (s *SSPI) shouldAuthenticate(req *http.Request) (shouldAuth bool) {
 
 // newUser creates a new user object for the purpose of automatic registration
 // and populates its name and email with the information present in request headers.
-func (s *SSPI) newUser(username string, cfg *models.SSPIConfig) (*models.User, error) {
+func (s *SSPI) newUser(username string, cfg *sspi.Source) (*models.User, error) {
 	email := gouuid.New().String() + "@localhost.localdomain"
 	user := &models.User{
 		Name:                         username,
@@ -214,7 +218,7 @@ func stripDomainNames(username string) string {
 	return username
 }
 
-func replaceSeparators(username string, cfg *models.SSPIConfig) string {
+func replaceSeparators(username string, cfg *sspi.Source) string {
 	newSep := cfg.SeparatorReplacement
 	username = strings.ReplaceAll(username, "\\", newSep)
 	username = strings.ReplaceAll(username, "/", newSep)
@@ -222,7 +226,7 @@ func replaceSeparators(username string, cfg *models.SSPIConfig) string {
 	return username
 }
 
-func sanitizeUsername(username string, cfg *models.SSPIConfig) string {
+func sanitizeUsername(username string, cfg *sspi.Source) string {
 	if len(username) == 0 {
 		return ""
 	}