Add support for logging at different levels by Wasm guest

[anth0nyleung]

Currently we support minimal logging. If udf calls println!, it is logged with log level log and this could or could not show up on the client depending on the setting of client_min_messages.

CREATE OR REPLACE FUNCTION logging() RETURNS INTEGER
    IMMUTABLE STRICT
    LANGUAGE PLRUST AS
$$
    println!("print");
    eprintln!("eprint");
    Ok(Some(0))
$$;

postgres=# select logging();
LOG:  print

WARNING:  eprint

 logging 
---------
       0
(1 row)
postgres=#
postgres=# set client_min_messages = 'debug1';
SET
postgres=# select logging();
LOG:  print

WARNING:  eprint

 logging 
---------
       0
(1 row)

We can consider adding enhancement to support for logging at different levels

notice!('notice');
warning!('warning');
error!('error');
log!('log');

[Hoverbear]

We don't think we're pushing WASM more for now, might revisit though!