OpenID Connect Support and Strict Mode

[Cyberboss]

Closes #2097

🆑 Configuration
The new configuration version is 5.6.0.
Added Security:OpenIDConnect entries and Security:OidcStrictMode. See the README for details.
Security:OAuth:Keycloak has been deprecated. Please migrate to an OIDC provider.
Security:OAuth:TGForums has been removed as the service is no longer provided.
/🆑

🆑
Added OpenID Connect support. See the README for details.
TGForums OAuth has been removed as the service is no longer provided.
/🆑

🆑 REST API
Added oidcProviderInfos to server information model. These list a friendlyName, user definable themeColour, and user definable themeIconUrl for indicating how to display OIDC providers. The schemeKey property is how authentication is performed. Send users to /oidc/{schemeKey}/signin to start the OIDC flow. Users will be returned to a configured endpoint (/app by default) with the query parameter state set to oidc.{schemeKey} and either a code indicating a TGS bearer token to use or a user readable error.
Added oidcConnections to users, similar to oAuthConnections with the only difference being that the provider is replaced with a schemeKey string. Modifiable with the same right.
/:cl:

🆑 GraphQL API
Added OIDC related fields.
Renamed adminUserCannotOAuth to adminUserCannotHaveServiceConnection.
Renamed editOwnOAuthConnections to editOwnServiceConnections.
/:cl:

🆑 Nuget: API
Added UserApiBase.OidcConnections.
Renamed ErrorCode.AdminUserCannotOAuth to AdminUserCannotHaveServiceConnection.
Renamed AdministrationRights.EditOwnOAuthConnections to EditOwnServiceConnections.
Made deprecated OAuthProviders Obsolete.
Added OidcProviderInfos and OidcStrictMode to ServerInformationResponse.
/:cl:

🆑 Nuget: Client
Updated to API library version 18.0.0.
/:cl:

TODO:

• Server deny password change in strict mode
• Server deny OAuth change in strict mode
• Server deny group changes in strict mode
• Webpanel handle OAuth in strict mode.
• Webpanel handle Password Change in strict mode.
• Webpanel handle group changes in strict mode.
• README