Implement read access branch permissions

[timsehn]

A customer asked for read access permissions on branches. Currently, branch permissions only control write access.

Branch read permissions would be useful to prevent data exfiltration in the customer per branch use case. This use case may become more popular because there is some potential to use it to prevent agents from having access to all customer's data, only a single customer's data.

[timsehn]

This is complicated because if you have access to the commit log, you can create a new branch that you can read from from a potentially restricted commit on another branch.

[timsehn]

It is also fine to have the access controls only exist for SQL Server, not the built-in remote server.