Source Generators: Allow embedding of files other than source

[chsienki]

We've seen from several customers the want to embed other files into the output assembly, particularly EmbeddedResources (see #49923)

We should explore an API that allows generators to add 'other' inputs to the compilation other than just source. This could be scoped explicitly to EmbeddedResources, or we might want to allow e.g. AdditionalFiles too.

Expanding it out to more types might have implications for analyzers, as they would be able to see them as well, but given that analyzers today see the generated source this seems desirable (although we might need an API that returns the provenance of the thing, such as Generated or Not).

[lsoft]

Could I ask to add a possibility to include a non-compilable additional files? I need to send some metadata from SG to my VSIX (and update custom codelenses with that metadata), so I need to access that metadata (for example XML file) in Compilation, but I don't need to have that XML in the binaries (so, it is not a resource, not an embedded resource, but something like BuildAction=None in Property window).

[ChrML]

This would be great. Source generators should be able to add other types of files to the output. One example would be Swagger that could generate the OpenAPI file at compile time and add the resulting .json to the compile output. Unless there is another way that could be done?

[elv1s42]

Would be very useful!
Another example is to use source generators for generation of some partial views like .cshtml or .razor files.