Ability to open a read-only Postgres connection

[radeusgd]

Add a flag to Postgres.Server constructor specifying to open a connection in read-only mode.

It should:

• block operations that modify data by a dialect flag, returning user-friendly errors,
• set a setting on the JDBC connection to disallow modifications in custom queries.

Should creating and modifying temporary tables be allowed? In the first iteration probably not, but overall it might be useful.

Once #9732 are implemented, the data link will delegate the read-only status to this connection flag.