pgwire: Add `SEVERITY_NONLOCALIZED` error field

[dvarrazzo]

Is your feature request related to a problem? Please describe.

Since version 9.6, PostgreSQL emits a SEVERITY_NONLOCALIZED field with its error messages (code is V). This field is missing in CRDB.

>>> import psycopg
>>> conn = psycopg.connect("postgresql://root@localhost:26257/defaultdb?sslmode=disable")
>>> try:
...     conn.execute("select wat")
... except Exception as e:
...     ex = e

>>> ex.diag.severity
'ERROR'  # This might be in another language

>>> ex.diag.severity_nonlocalized
None

Describe the solution you'd like

Please introduce this field, as it might be used by clients to decide what to do with a notification.

It needs to be added here:

cockroach/pkg/sql/pgwire/conn.go

Line 1475 in daf3b48

msgBuilder.writeTerminatedString(pgErr.Severity)

Jira issue: CRDB-16071

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