Request: Support GSSAPI authentication

This is more of a feature request than a bug report, but:

## What did you do?

- I set up a connection favourite to a Postgres server that requires GSSAPI authentication (https://www.postgresql.org/docs/10/static/auth-methods.html#GSSAPI-AUTH) and tried to connect to it.

## What did you expect to happen?

- I expected to be able to authenticate to the server using an existing Kerberos ticket I had on my client machine.

## What actually happened?

- I saw an error message on the connection favourite: "GSSAPI authentication not supported"

## What software versions are you using?

Postico version: Version 1.3.2 (2318)

macOS version: 10.12.6

PostgreSQL version: 10

Start by tracing how Postico handles connection favourites and where the “GSSAPI authentication not supported” error is produced. Test against a PostgreSQL 10 server requiring GSSAPI, using an existing Kerberos ticket, and consider the work complete when the connection authenticates successfully.

Written by the indexing model from the issue text.