Add support for applying roles based on Github sponsorship

Identified this GraphQL Github query:

```graphql
query($user:String!) {
user(login:$user) {
sponsors(first:20) {
nodes {
... on User {
login
}
}
pageInfo {
endCursor
startCursor
}
totalCount
}
}
}
```

and [webhook info](https://docs.github.com/en/sponsors/integrating-with-github-sponsors/configuring-webhooks-for-events-in-your-sponsored-account).

Start by locating the existing GitHub user-linking and contributor-role application flow in the bot. Review the supplied GitHub Sponsors GraphQL query and webhook documentation, then determine how sponsorship data should reach that flow. Done means the issue's requested sponsor-based role behavior is implemented and verified against the relevant GitHub events.

Written by the indexing model from the issue text.