MSC Multi-Authoring

Is your feature request related to a problem? Please describe.

Sometimes, MSCs will gain or have multiple authors. Be it in a corporate/group setting where multiple people have to interact with reviews and threads from the SCT or community. Or in a delegate/"community shepherd" setting where the original author does not have the time or energy to continue working on a MSC.

In these situations, people can still contribute to the MSC, but they crucially do not have the same capabilities as the original PR author, as the following;

• (Un)Resolving threads
• Editing the PR description
  • Often required to update implementations, but other reasons may also exist.
• Update the PR branch itself with commits.

Describe the solution you'd like.

For the first two issues, I'd like to request that the mscbot is able to respond to commands such as @mscbot resolve (on a thread) or @mscbot update_description <new description> (as a comment, which then gets hidden after processing).

For the last issue, there is #21.

To activate it, the original author should add Author: @user_a, @user_b, ... to the PR description. (Obviously, the update_description will not be able to overwrite this part)

Start at mscbot's handling of pull-request comments and review threads, then trace how pull-request descriptions and author permissions are represented. The issue specifies commands for resolving threads and updating descriptions, plus an Author declaration that must remain protected. Done means authorized co-authors can perform the requested actions while unauthorized users cannot.