Add named shadows support

[gregbreen]

Issue #, if available:
#55

None

Description of changes:

Added support for named shadows. Required for a project I'm working on.

The major design choice was the decision to parameterize (with new shadowName parameter) all of the existing shadow macros, rather than make a new set of macros for named shadows.

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[gregbreen]

@sarenameas @leegeth I synchronized the branch with latest main and resolved the merge conflicts. So this is again merge-able. I hope you may be able to look at this soon. I'm happy to address comments over Xmas/NY if you can make comments before Xmas.

[gregbreen]

Per some offline conversation, I updated the PR to retain the old API functions and function-like macros for backwards compatibility.

I wasn't keen to essentially duplicate all of those macros, so we could have a second set that each added a shadowName argument. Also, I couldn't really do that anyway because the long macro names were already exceeding 31 characters; longer (new) names contravene MISRA Rule 5.1, requiring external identifiers to be distinct in the first 31 characters.

So instead I've collapsed the API down to pretty much just two function-like macros (one for length computation and one for string assembly), with these accepting the operation as an extra argument (instead of a specialized macro for each operation). Backwardly-compatible macros marked as deprecated.

I spent days thinking about the semantics on this. Not sure it's palatable to others, but I feel confident it's better than before (as well as retaining backwards compatibility). I won't update the main device SDK PR (shadow demo and integration tests), until or unless this API is accepted.

Note: Doxygen pipeline check is failing because Doxygen is not installing. Nothing to do with this PR I believe.

[gregbreen]

OK. After considerable additional offline conversation, re-worked the backwards compatibility.

• Full support of all existing macros.
• Retain the concept of one convenience function-like macro for each operation-suffix. (but now with a shadowName parameter)
• Ensure all new macros names are 31 characters or fewer (to ensure no issues with MISRA 5.1).

New function example (with shadowName parameter):

Deprecated function example:

New function-like macro example (with shadowName parameter):

Deprecated function-like macro example:

Note: The CI/CD Doxygen check is still failing. It's because https://www.doxygen.nl/files/doxygen-1.8.20.linux.bin.tar.gz is giving a 404. Looks like this will affect lots of the SDK repos.