C, parse attributes

[jakobandersen]

Feature or Bugfix

• Feature
• Bugfix

The 3.0 version of the C domain got more strict, resulting in warnings for declarations that are not technically C, but are rather common, so it would be nice to release this PR soon.

Purpose

Parse attributes in declarations and allow users to define their own identifiers to be parsed as attributes (e.g., when the actual attribute is hidden behind a macro). For example:

.. c:function:: LIGHTGBM_C_EXPORT int LGBM_BoosterFree(int handle)

Here the LIGHTGBM_C_EXPORT identifier is a macro that expands to a symbol visibility attribute.

Relates

• Error in declarator or parameters breathe-doc/breathe#500

[tk0miya]

I'm not good at the implementation of C/C++ domain. So I can't review this sooner.
But I can say okay to merged this into 3.0.x branch if you worried about merging "enhancement" changes into the branch.

[jakobandersen]

But I can say okay to merged this into 3.0.x branch if you worried about merging "enhancement" changes into the branch.

Thanks, I'll merge it as soon as the CI goes through.