Add http file showcasing how to use the management API

[kzu]

No public client for now since it deviates from the gRPC protocol for everything else. Perhaps we'll introduce a typed API at some point?

To send the requests, users should create an http-client.env.json alongside the http file with the following content:

{
  "management": {
    "teamId": "...",
    "token": "...",
    "userKey": "[after sending the create key request]",
    "userKeyId": "[after sending the create key request]"
  }
}

[kzu]

🧪 Details on Ubuntu 24.04.3 LTS

❌ xAI.Tests.SanityChecks.IntegrationTestStreaming

Assert.NotEmpty() Failure: Collection was empty
   at xAI.Tests.SanityChecks.AssertIntegrationTest(ChatResponse response, Func`1 getDateCalls) in /_/src/xAI.Tests/SanityChecks.cs:line 299
   at xAI.Tests.SanityChecks.IntegrationTestStreaming() in /_/src/xAI.Tests/SanityChecks.cs:line 180

🧪 Details on Ubuntu 24.04.3 LTS

from retest v1.1.0 on .NET 10.0.1 with 💜 by @devlooped