Support shared client themes in messages

[freya022]

General Troubleshooting

• I have checked for similar issues on the Issue-tracker.
• I have updated to the latest JDA version
• I have checked the branches or the maintainers' PRs for upcoming features.

Feature Request

JDA should support reading shared client themes as well as sending them (but not edit).

Discord docs changes: discord/discord-api-docs@d891f0e
Follow-up: discord/discord-api-docs@bb9449b

I am posting this issue to track this feature.

Note: Remember to update isV1Valid/isV2Valid and buildV1/buildV2 in MessageCreateBuilder with the new constraints.

If you want to pick this up, please let us know in the #lib-dev channel of the Discord server.

Example Use-Case