feat(pubsub): implement into_stream for a Session

[haphungw]

Allow Session to be used as a futures::Stream. This enables the use of streaming primitives for message processing.
This feature is gated behind unstable-stream to maintain API stability.

Fixes #4574

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (4662c64) to head (c1e4461).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4590   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         195      195           
  Lines        7458     7464    +6     
=======================================
+ Hits         7086     7092    +6     
  Misses        372      372           

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[dbolduc]

The code all looks good.

• we definitely need to exercise the new feature in our CI
• we should document the new feature
• the tests could be cleaned up a bit

[dbolduc]

Just the docs thing, then I think we are good to go

[dbolduc]

This works?? I think it should be:

Suggested change

	#[cfg_attr(docsrs, feature(doc_cfg))]
	#![cfg_attr(docsrs, feature(doc_cfg))]

basically #[...] applies to the thing immediately below it, but #![...] applies to the module in scope, which in lib.rs, is the whole crate.

[dbolduc]

Ahh lol, we only try to build 20 crates on PRs (probably for timing reasons). I guess pubsub is not one of them.

google-cloud-rust/.gcb/scripts/docs-rs.sh

Lines 25 to 28 in 6e053dd

	packages=("${all_packages[@]:0:20}")
	if [[ "${TRIGGER_NAME:-}" == "gcb-pm-*" ]]; then
	packages=("${all_packages[@]}")
	fi

I think the post-merge build would fail.