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)