Use the HTTP_V2 alongside LegacyHTTP

What does this MR do and why?

• Move the existing HTTP lib to LegacyHTTP
• Use the existing HTTP file to switch between HTTP and LegacyHTTP via the new feature flag use_gitlab_http_v2
• Adjust test files according to the new changes

These changes are behind the feature flag use_gitlab_http_v2 #426549 (closed).

Issue / MR	Link
Main issue	#415686 (closed)
First MR to initialize the gem	!125024 (merged)
Small fixes	!131612 (merged)
POC	!131644 (closed)
Enabling on GitLab	!132238 (merged)
Replace UrlBlocker::BlockedUrlError with HTTP_V2	!132541 (merged)
Use the HTTP_V2 alongside LegacyHTTP	==HERE== !132742 (merged)

Related to #415686 (closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.