[libwebm] Add export config

[jimwang118]

Fixes #42034

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download.
• The "supports" clause reflects platforms that may be fixed by this new version.
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

Usage test passed with x64-windows triplet.

[dg0yt]

Portname: libwebm
Required package name: unofficial-libwebm
Required target prefix: unofficial::libwebm::

If you want "symmetry" in the target name, i.e. unofficial::libwebm::libwebm, you can do set_target_properties(webm PROPERTIES EXPORT_NAME libwebm).

The extra config file is pointless at the moment. Just change the install(EXPORT ... FILENAME ...) to write -config.cmake.

[d1str4ught]

when will this be merged so I can use libwebm?

[dg0yt]

This can't be merged as long it doesn't implemented the maintainer guidelines (and passed review).
This doesn't mean you couldn't use libwebm. It is just less convenient.

[dg0yt]

And remember to offer it upstream.

[jimwang118]

And remember to offer it upstream.

Ok，A PR 6 has been submitted upstream.