Mutations request to the Explicit Permissions API

Created by: tobiasbrodersen

Feature request description

GraphQL mutation that can append or remove a single user from a repository userPermissions list.

From reading the documentation and searching through the api console, the only mutation available to manipulate users permissions is setRepositoryPermissionsForUsers

Is your feature request related to a problem? If so, please describe.

In our specific case we have multiple code hosts that use different mean of authentication, this unfortunately means we can't utilize background syncing.

Instead we're looking to implement a middleware service that can reflect teams and individuals on SourceGraph by the means of the Explicit Permissions API.

Having a mutation that can append a single user would reduce the complexity of above mentioned service a lot

Additional context

Repositories: 7000+ across multiple code hosts SourceGraph seats: 1000