feat: Add AsyncHTTPIterator to page_iterators_async

[kiraksi]

This PR:

• Creates an AsyncHTTPIterator that can be used for async iterators in client libraries, in bigquery it will be used as the parent class of AsyncRowIterator

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #627 🦕