azp: add feature flag endpoint

[ChristopherHX]

[2024-06-14 22:58:28Z WARN FeatureFlagProvider] Unable to retrieve feature flag status: Microsoft.VisualStudio.Services.WebApi.VssResourceNotFoundException: API resource location 3e2b80f8-9e6f-441e-8393-005610692d9c is not registered on http://192.168.1.2:36837/.
   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.CreateRequestMessageAsync(HttpMethod method, IEnumerable`1 additionalHeaders, Guid locationId, Object routeValues, ApiResourceVersion version, HttpContent content, IEnumerable`1 queryParameters, Object userState, CancellationToken cancellationToken, String mediaType)
   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.SendAsync[T](HttpMethod method, IEnumerable`1 additionalHeaders, Guid locationId, Object routeValues, ApiResourceVersion version, HttpContent content, IEnumerable`1 queryParameters, Object userState, CancellationToken cancellationToken)
   at Agent.Listener.Configuration.FeatureFlagProvider.GetFeatureFlagWithCred(IHostContext context, String featureFlagName, ITraceWriter traceWriter, AgentSettings settings, VssCredentials creds, CancellationToken ctk) in /mnt/vss/_work/1/s/src/Agent.Listener/Configuration/FeatureFlagProvider.cs:line 62

As long this is not implemented we cannot turn these restricted features on and get a bunch of warnings

Changelogs mention the implementation: https://github.com/microsoft/azure-pipelines-agent/releases?q=Featureflag&expanded=true