postgresql: Initial parser implementation

This adds a protocol parser for the PostgreSQL protocol and a new
postgresql.log similar to the existing mysql.log.

This should be considered preliminary and hopefully during 7.1 and 7.2
with feedback from the community, we can improve on the events and logs.
Even if most PostgreSQL communication is encrypted in the real-world, this
will minimally allow monitoring of the SSLRequest and hand off further
analysis to the SSL analyzer.

This originates from github.com/awelzel/spicy-postgresql, with lots of
polishing happening in the past two days.

testing/btest/scripts/base/protocols/postgresql/startup-parameter.zeek

@@ -0,0 +1,12 @@
+# @TEST-DOC: Event for name, value pairs in the startup message.
+#
+# @TEST-REQUIRES: ${SCRIPTS}/have-spicy
+# @TEST-EXEC: zeek -b -Cr ${TRACES}/postgresql/psql-login-no-sslrequest.pcap %INPUT >output
+#
+# @TEST-EXEC: btest-diff output
+
+@load base/protocols/postgresql
+
+event PostgreSQL::startup_parameter(c: connection, name: string, value: string) {
+	print "startup_parameter", c$uid, name, value;
+}