Re: Proposal: allow database-specific role memberships

In building off of prior art regarding the 'pg_read_all_data' and 'pg_write_all_data' roles, I would like to propose an extension to roles that would allow for database-specific role memberships (for the purpose of granting database-specific privileges) as an additional layer of abstraction.

= Problem =

There is currently no mechanism to grant the privileges afforded by the default roles on a per-database basis. This makes it difficult to cleanly accomplish permissions such as 'db_datareader' and 'db_datawriter' (which are database-level roles in SQL Server that respectively grant read and write access within a specific database).

The recently-added 'pg_read_all_data' and 'pg_write_all_data' work similarly to 'db_datareader' and 'db_datawriter', but work cluster-wide.