alts: Read max number of concurrent ALTS handshakes from environment variable.

[matthewstevenson88]

This mimics the existing functionality for Java (https://github.com/grpc/grpc-java/pull/10016) and for C++ (https://github.com/grpc/grpc/pull/32672). When the environment variable is not set, the functionality is unchanged.

Along the way, we remove 2 obsolete TODOs in handshaker.go.

RELEASE NOTES:

• alts: add support for GRPC_ALTS_MAX_CONCURRENT_HANDSHAKES env var

[matthewstevenson88]

@easwars Would you be able to take a look at this PR when you have a chance? :)

[github-actions[bot]]

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[matthewstevenson88]

Waiting on #6300 before coming back to this.

[dfawley]

#6300 is merged; is this ready to work on again?

[matthewstevenson88]

#6300 is merged; is this ready to work on again?

Yes thanks! I've been cleaning it up this morning. Let me amend the tag.

[matthewstevenson88]

Thanks for the great suggestions @easwars and apologies for the delay: your suggestion to use a weight semaphore was a great one, but I wanted to add another e2e test to ensure correctness and it took a little while to get it working as expected. :)

[matthewstevenson88]

@dfawley It seems @easwars is OOO, would you mind if I re-assign to you to finish the review?

[matthewstevenson88]

Why are all these go.mod changes needed? That seems unexpected, given that all the code changes are in the main module.

Sorry about that, not sure what happened there. This should all be cleaned up now.

[matthewstevenson88]

Thanks for the review!