fix(studio): Allow setting UART RPC thread priority

[petejohanson]

To ensure we can tune things when other threads may have priority, preventing UART processing in time for the studio UI requirements, adjust our default UART thread priority, and allow overriding as needed.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.