feat: add ability for users to convert their password login type to oauth/github login (#8105)

* Currently toggled by experiment flag

---------

Co-authored-by: Bruno Quaresma <bruno@coder.com>

cli/server.go

@@ -683,6 +683,39 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 				}
 
 				options.AppSecurityKey = appSecurityKey
+
+				// Read the oauth signing key from the database. Like the app security, generate a new one
+				// if it is invalid for any reason.
+				oauthSigningKeyStr, err := tx.GetOAuthSigningKey(ctx)
+				if err != nil && !xerrors.Is(err, sql.ErrNoRows) {
+					return xerrors.Errorf("get app oauth signing key: %w", err)
+				}
+				if decoded, err := hex.DecodeString(oauthSigningKeyStr); err != nil || len(decoded) != len(options.OAuthSigningKey) {
+					b := make([]byte, len(options.OAuthSigningKey))
+					_, err := rand.Read(b)
+					if err != nil {
+						return xerrors.Errorf("generate fresh oauth signing key: %w", err)
+					}
+
+					oauthSigningKeyStr = hex.EncodeToString(b)
+					err = tx.UpsertOAuthSigningKey(ctx, oauthSigningKeyStr)
+					if err != nil {
+						return xerrors.Errorf("insert freshly generated oauth signing key to database: %w", err)
+					}
+				}
+
+				keyBytes, err := hex.DecodeString(oauthSigningKeyStr)
+				if err != nil {
+					return xerrors.Errorf("decode oauth signing key from database: %w", err)
+				}
+				if len(keyBytes) != len(options.OAuthSigningKey) {
+					return xerrors.Errorf("oauth signing key in database is not the correct length, expect %d got %d", len(options.OAuthSigningKey), len(keyBytes))
+				}
+				copy(options.OAuthSigningKey[:], keyBytes)
+				if options.OAuthSigningKey == [32]byte{} {
+					return xerrors.Errorf("oauth signing key in database is empty")
+				}
+
 				return nil
 			}, nil)
 			if err != nil {