MSC Multi-Authoring

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#### 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 ` (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)

Inizia dalla gestione da parte di mscbot dei commenti sulle pull request e dei thread di revisione, quindi traccia il modo in cui sono rappresentati le descrizioni delle pull request e i permessi degli autori. L'issue specifica i comandi per risolvere i thread e aggiornare le descrizioni, oltre a una dichiarazione Author che deve rimanere protetta. Il lavoro è completato quando i coautori autorizzati possono eseguire le azioni richieste, mentre gli utenti non autorizzati non possono farlo.

Scritto dal modello di indicizzazione a partire dal testo della issue.