Added filter for request path

[adrianosferreira]

Description

This fix is needed so third party plugins will be able to append URL arguments on internal requests, as Gutenberg concatenate the rest_url avoiding us to filter it on PHP side.

Types of changes

New feature (non-breaking change which adds functionality)

Issue: #5958

[danielbachhuber]

This seems like a very liberal filter whose abuse could lead to some unexpected results further down the road. Can you share how you would use it?

[adrianosferreira]

@danielbachhuber something like this:

addFilter(
	'request.path',
	path,
	path => {
		const params = new URLSearchParams(path);
		return params.append( 'lang', icl_vars.current_lang );
	}
);

[danielbachhuber]

@adrianosferreira Makes sense, thanks for clarifying.

@WordPress/gutenberg-core Have an opinion on this? I'm leaning towards 👎 because I think it's too liberal of a filter, but I don't have a reasonable alternative to suggest.

[youknowriad]

Also, this is limited to withAPIData and we're in the process of refactoring these entirely in favor of core-data resolvers.

[danielbachhuber]

@adrianosferreira Let's hold off on this particular change for now, and we can make sure we discuss #5958 again as a part of the merge proposal process