Add embyupdate userid config option

[shoriminimoe]

Description

Fixes #4402.

Adds a userid config option for the embyupdate plugin. When the
userid is provided, the Emby user lookup is bypassed. This avoids a
failure to get the user ID for private users.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[shoriminimoe]

One of the original requests from #4402 was to change what is returned by get_user. I chose not to change that because that function returns the JSON object for the user from the Emby api and not just a username and ID. Instead, the user lookup is skipped entirely if a userid is supplied in the config.

[sampsyo]

Nice; this looks perfect! Many thanks!!