Fix git push and remote operations failing on non-HTTPS remote URLs

[eranturgeman]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.
• Update documentation about new features / new supported technologies

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Fix git operations failing when .git/config remote URL is not HTTPS

Push, BranchExistsInRemote, and RemoveRemoteBranch all resolved the remote URL by reading from .git/config via the remote name. This works when the repo was cloned over HTTPS, but fails in two scenarios:

1. Bitbucket Pipelines CI — the pipeline agent overwrites the remote URL to an internal proxy URL (http://bitbucket.org/...) before the user script runs. go-git bypasses the pipeline's git proxy, so pushing to that URL directly fails with "authentication required".
2. SSH-cloned repos — when a repo is cloned via SSH, .git/config stores a git@... URL. go-git detects SSH transport, sees HTTP auth credentials, and returns ErrInvalidAuthMethod. (SSH is not supported in Frogbot but this is a start for it)

GitManager already has remoteGitUrl which is guaranteed to be HTTPS — SetRemoteGitUrl explicitly normalizes any non-HTTPS URL to the HTTPS URL returned by the VCS provider API. Fetch was already using RemoteURL: gm.remoteGitUrl correctly; Push, BranchExistsInRemote, and RemoveRemoteBranch inconsistently were not.

The fix makes all three use gm.remoteGitUrl for the actual network connection, consistent with how Fetch already works.