Correlation id utilities (GSI 501)

[TheByronHimes]

This adds the correlation ID utilities as outlined in the Marabou Stork epic.
This PR only adds the utilities and does not modify the behavior of the Kafka providers (that will come next).
The new stuff:

Functions

• get_correlation_id()
• new_correlation_id()
• set_correlation_id()
• validate_correlation_id()
• set_context_var()

Errors

• CorrelationIdContextError
• InvalidCorrelationIdError
• MissingCorrelationIdError
  I think MissingCorrelationIdError could be folded into the InvalidCorrelationIdError as it's only used in set_correlation_id to differentiate between an empty string vs an invalid non-empty string.

Other

• correlation_id_var: The ContextVar instance that is used to store/access a correlation ID for a context.

[mephenor]

Looks nice. Personally I'd fold the CorrelationIDContext and MissingContextID errors into one, don't see a reason to keep them separate if we don't necessarily need to distinguish between unset and empty ID.

[coveralls]

Pull Request Test Coverage Report for Build 7099687386

• 44 of 45 (97.78%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 90.157%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/hexkit/correlation.py	31	32	96.88%

Totals
Change from base Build 6960935059:	0.3%
Covered Lines:	1374
Relevant Lines:	1524

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💛 - Coveralls

[TheByronHimes]

@mephenor I've consolidated the errors then.

[Cito]

Looks good, just some suggestions regarding the testing.

[TheByronHimes]

@Cito please see the changes in 24dce0f and resolve the conversations if you are satisfied. I'm happy to make further modifications if needed.

[Cito]

Thanks for the changes.