Allow suppressing warnings

[aeisenbarth]

It would be great being able to ignore warnings like WARNING: Cannot resolve forward reference in type annotations of…
(In reference to sphinx-doc/sphinx#10219 (comment))

The reason is that some warnings are due to limited capabilities of the extension, and cannot always be resolved with reasonable effort. I would think (sarcastically), "if you can't support a feature reliably, don't implement it". Practically, I would interpret that as make such features optional or their warnings silenceable.

This is relevant because when users are flooded with hundreds of irrelevant warnings, they cannot detect and resolve relevant warnings. They can also not turn warnings into errors to enforce fixing them.

I had no success using Python's standard warnings filter mechanism (Sphinx warnings seem not accessible by it).
Inspite of extensions not being covered by default by suppress_warnings (sphinx-doc/sphinx#10219), autodoc is able to provide custom warning types autodoc, autodoc.import_object.