New self-hosted config option platformOptions

[RahulGautamSingh]

Describe the proposed change(s).

To improve the organization and accessibility of platform-specific configuration options, combine them under a single config option named platformOptions.

Example:

{
  "platformOptions": {
    "gitlabMergeCheckAttempts": 3,
    "gitlabBranchStatusDelay": 4,
    "gitlabMergeRequestDelay": 100,
    "githubTokenWarn": false,
    "gitLabIgnoreApprovals": true,
    "bbUseDevelopmentBranch": true,
  }
}

Originally posted by @viceice in #29137 (comment)

[RahulGautamSingh]

Should azureWorkItemId, bbUseDefaultReviewers be included in this list too? These 2 are a repo config options, whereas all the others options that I included in the pr description are global only options.

[viceice]

Should azureWorkItemId be included in this list too? It is a repo config option currently whereas all the others are already global only options.

I'm not sure about that 🤔