[Enhancement]: Better github access scopes for bigger teams

[regdocs]

Affected Area

None

Problem Statement

When storing GitHub access token of a user in the team document, the token is accessible to all members of the team, and so are all the private repositories accessible to the token.

Proposed Solution

There should be better scoping in terms of who is allowed to have whitelisted access to a given repository.

ref: https://support.frappe.io/helpdesk/tickets/74797