Add async support to gitlab-http

What does this MR do and why?

After the initial implementation of the gitlab-http gem (#415686 (closed)), we'll add the async support to it. Related to #427701 (closed).

When using the gitlab-http gem, you can pass the async: true option and get a LazyResponse object. Since there is no business logic and external connection inside the async calls, it is thread-safe.

Screenshots or screen recordings

locations = [
  'https://hub.dummyapis.com/delay?seconds=1',
  'https://hub.dummyapis.com/delay?seconds=2',
  'https://hub.dummyapis.com/delay?seconds=3',
  'https://hub.dummyapis.com/delay?seconds=4',
  'https://hub.dummyapis.com/delay?seconds=5'
]; nil

promises = locations.map do |location|
  Gitlab::HTTP_V2.get(location, async: true)
end; nil

promises.each(&:execute).each(&:wait); nil

puts promises.map(&:value).map(&:body)

After ~5 seconds, you'll see this;

This response has been delayed for 1 seconds
This response has been delayed for 2 seconds
This response has been delayed for 3 seconds
This response has been delayed for 4 seconds
This response has been delayed for 5 seconds

MR acceptance checklist

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