Implement read access branch permissions

A customer asked for read access permissions on branches. Currently, [branch permissions](https://docs.dolthub.com/sql-reference/server/branch-permissions) only control write access.

Branch read permissions would be useful to prevent data exfiltration in the [customer per branch](https://www.dolthub.com/blog/2025-05-08-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.