Add set_config and the transaction mode checks after a query

[tamnd]

This PR adds set_config, which is SET as a function, and the rule that a transaction cannot change its isolation level after its first query. With it, the session item of M2 is complete.

What changes

• rupg-func has the kernel of set_config_by_name. A null name gives 22004, a null value resets the parameter, and a null is_local is false, as in guc_funcs.c.
• The Session trait of rupg-func has set_setting. The reader of rupg-session changes the settings of the connection through a RefCell, with the same checks as SET, the check of role and session_authorization included. The change counts for the rest of the query. The zone of TimeZone is made again when the setting changes, so Session::zone now gives an Rc.
• An error or a rollback undoes the change through the stack of the transaction that SET already uses, and a local change ends with its transaction. A changed reported parameter, such as TimeZone, DateStyle or search_path, is reported before ReadyForQuery.
• Settings knows when the transaction took its first snapshot, as FirstSnapshotSet does. A query takes it when it is analyzed in a Query or a Parse message, and in Bind and Execute. After it, check_transaction_isolation, check_transaction_deferrable and check_transaction_read_only give 25001. SHOW and the other utility statements take no snapshot.

Tests

• The new test set_config sends 44 queries to a connection and checks the rows, the errors and the ParameterStatus messages. Each expected value is the output of the PostgreSQL 19 oracle for the same statement.
• The new test transaction_modes_after_a_query checks SET TRANSACTION after a query in a block, in an implicit block and after Parse, against the oracle.
• cargo fmt, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace and the cargo xtask checks pass on server1.