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/interface.go

@@ -5,6 +5,7 @@
 package auth
 
 import (
+	"context"
 	"net/http"
 
 	"code.gitea.io/gitea/models"
@@ -18,18 +19,8 @@ type DataStore middleware.DataStore
 // SessionStore represents a session store
 type SessionStore session.Store
 
-// Auth represents an authentication method (plugin) for HTTP requests.
-type Auth interface {
-	Name() string
-
-	// Init should be called exactly once before using any of the other methods,
-	// in order to allow the plugin to allocate necessary resources
-	Init() error
-
-	// Free should be called exactly once before application closes, in order to
-	// give chance to the plugin to free any allocated resources
-	Free() error
-
+// Method represents an authentication method (plugin) for HTTP requests.
+type Method interface {
 	// Verify tries to verify the authentication data contained in the request.
 	// If verification is successful returns either an existing user object (with id > 0)
 	// or a new user object (with id = 0) populated with the information that was found
@@ -37,3 +28,33 @@ type Auth interface {
 	// Returns nil if verification fails.
 	Verify(http *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) *models.User
 }
+
+// Initializable represents a structure that requires initialization
+// It usually should only be called once before anything else is called
+type Initializable interface {
+	// Init should be called exactly once before using any of the other methods,
+	// in order to allow the plugin to allocate necessary resources
+	Init() error
+}
+
+// Named represents a named thing
+type Named interface {
+	Name() string
+}
+
+// Freeable represents a structure that is required to be freed
+type Freeable interface {
+	// Free should be called exactly once before application closes, in order to
+	// give chance to the plugin to free any allocated resources
+	Free() error
+}
+
+// PasswordAuthenticator represents a source of authentication
+type PasswordAuthenticator interface {
+	Authenticate(user *models.User, login, password string) (*models.User, error)
+}
+
+// SynchronizableSource represents a source that can synchronize users
+type SynchronizableSource interface {
+	Sync(ctx context.Context, updateExisting bool) error
+}