Add support to plpgsql for postgres' GET DIAGNOSTICS

**Is your feature request related to a problem? Please describe.**
In procs/udfs, I tried to use `GET DIAGNOSTICS rc = row_count;`

**Describe the solution you'd like**
I'd like to support Postgres' implementation of this feature.

```
jim@azure.auracoda.com:26257/jim> create function fn_t1_insert (name varchar) returns int as
-> $$
-> declare
-> rc INT := 0;
-> begin
-> insert into t1 ( name ) values ( name );
->
-> GET DIAGNOSTICS rc = ROW_COUNT;
-> return rc;
-> end
-> $$ language PLpgSQL;
->
ERROR: unimplemented: attempted to use a PL/pgSQL statement that is not yet supported
SQLSTATE: 0A000
HINT: You have attempted to use a feature that is not yet implemented.

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```

**Additional context**
I'm using 23.2 RC1

Jira issue: CRDB-35161

Epic CRDB-66132