Support for player count, uptime, GitHub App authentication, and git credentials validation

[Cyberboss]

🆑
GitHub and GitLab credentials for instances are now validated when created/updated.
The host watchdog no longer needs to be launched with a special parameter to enable SystemD interoperability.
/:cl:

🆑 HTTP API
HTTP 424 will be returned during PUT or POST /api/Repository if provided credentials are determined to be invalid.
Added error code 108 for if a repository accessToken was determined to be for a user different to accessUser.
Repository access credentials can now use GitHub App private keys in a special encoded format: "TGS_PK_" + (APP_ID OR CLIENT_ID) + ":" + BASE64(APP_PRIVATE_KEY) (Clients should perform this formatting for users). If this is used, the app name should be set as the username. Required permissions are contents: read for cloning, contents: write for pushing test merge commits, pull_requests: write for posting test merge comments, and contents: write + deployments: write for enabling GitHub deployments.
Added ISO8601 launchTime field to DreamDaemon response model.
Added clientCount field to DreamDaemon response model. Requires game side interop version >= 5.10.0 and watchdog health checks to be enabled to function.
/:cl:

🆑 Nuget: API
BREAKING: Moved RepositorySettings out of Internal namespace and made abstract.
Added TGS encoded app private key prefix to RepositorySettings.
Added DreamDaemonApiBase.LaunchTime.
Added DreamDaemonApiBase.ClientCount.
/:cl:

🆑 Nuget: Client
BREAKING: Updated API definitions library to 14.0.0.
/:cl:

🆑 DreamMaker API
Added support for relaying current client count back to TGS during health checks.
/:cl:

Closes #1909
Closes #1858
Closes #1876