Add phpstub for amqp extension (php-amqp)

[mkilmanas]

Disclaimer: first time contributor here

This adds phpstub file for AMQP extension by copying contents of their originally provided stubs and concatenating into a single file.

What I am uncertain of and would like your feedback on:

• Should stubs in psalm contain original docblocks with explanations? Or only those with annotations that help with types, deprecations, etc?
• I have added a link to their LICENCE file (copying the whole file over seems a bit of an overkill). Is that sufficient?
• I have found two places to register/allow the new extension - have I missed any others? It did not seem like any tests would be specifically checking for each extension, so no new tests added either.

Looking forward to your comments (and hopefully a merge at the end)

[mkilmanas]

Hey, what's the status and overall position with this PR - is it at all worth pursuing further?

2 checks failed for debatable reasons:
a) The PR does not have any labels assigned, which I am not in position to add myself, so it's up for maintainers, I suppose
b) Backwards compatibility check is failing, but I find the actual reason to be extremely formal and cannot think of any real-life situation where that could actually break anything or cause any other kind of trouble.

Please let me know what you think and anything that I could improve here.

[danog]

Thank you!