Add OAuth2 Client ID Metadata Document support

MCP connectors such as ChatGPT and Claude register via HTTPS
client_id URLs instead of pre-provisioned GIDs. Fetch and cache
their metadata documents, upsert clients on first use, and
advertise CIMD in OIDC discovery when allowed URLs are configured.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/probodconfig/auth_config.go

@@ -38,6 +38,7 @@ type OAuth2ServerConfig struct {
 	RefreshTokenDuration      int                      `json:"refresh-token-duration"`
 	AuthorizationCodeDuration int                      `json:"authorization-code-duration"`
 	DeviceCodeDuration        int                      `json:"device-code-duration"`
+	CIMDAllowedClientIDs      []string                 `json:"cimd-allowed-client-ids"`
 }
 
 type OAuth2SigningKeyConfig struct {