Adding application_name to the error and notice message fields

Hi!

We take care to always set application_name to improve our log lines
where we use %a in log_line_prefix to log application name, per [1].
But notices which are sent to the client do not have the application
name and are thus hard to attribute correctly. Could "a" be added with
the application name (when available) to the error and notice message
fields [2]?

Mitar

[1] https://www.postgresql.org/docs/current/runtime-config-logging.html#GUC-LOG-LINE-PREFIX
[2] https://www.postgresql.org/docs/current/protocol-error-fields.html

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