Support external SASL in AMQP 1.0 client URI

[carlsmedstad]

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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• 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
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Further Comments

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

Thanks!

[michaelklishin]

Thank you, @carlsmedstad @ansd.

[michaelklishin]

@carlsmedstad this can be considered a trivial change but we'd appreciate if you could sign our CLA (electronically, using Box Sign) for this and any future contributions. Thank you.

[carlsmedstad]

@michaelklishin As far as I'm aware I signed it before creating the PR, but I might've made a mistake.