Support external SASL in AMQP 1.0 client URI (backport #14863)

[mergify]

See discussion: #14775

Proposed Changes

This allows e.g. dynamic shovels to authenticate with brokers using external SASL authentication.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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This is simply a reminder of what we are going to look for before merging your code.

• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
• I have read the CONTRIBUTING.md document
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

Disclaimer: No Erlang experience and not familiar with the codebase - I might have very well missed something.

Thanks!

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This is an automatic backport of pull request #14863 done by Mergify.