fix: refine the session variables accessing (#193)

[VWagen1989]

This PR addresses the modification and retrieving problems of session variables under PostgreSQL protocol. I've ported the implementation of managing system variables of PostgreSQL from doltgresql. And now MyDuck server have all the PostgreSQL system variables.

In addition, this PR also solve these problems:

• Implementing the query intercepter that captures SET statement, RESET statement, SHOW statement and query on the function current_setting(). Both the system variables for PostgreSQL and DuckDB can be accessed as expected. P.S. SHOW ALL and RESET ALL are not supported yet.
• The statements listed above can be handled successfully under both simple query and extended query. Please see the test in pgserver/sess_params_test.go.

[fanyang01]

LGTM! This PR improve our PG compatibility a lot since many client libraries get/set the PG parameters on their first connection.