Updating drush config to allow extra-dump command for max_allowed_packet

[cdchris12]

We recently discovered that drush doesn't consume the MARIADB_MAX_ALLOWED_PACKET value, which can cause issues for environments with larger than usual tables. This PR adds that value to the drush.yml file, and adds a sane default of 64M which can be overwritten if needed.

[tobybellwood]

@cdchris12 I've changed this slightly to wrap the entire extra-dump option as a variable EXTRA_DUMP_CMD="--max_allowed_packet=64M" - this then allows it to entirely overridden downstream (e.g. in PostgreSQL), or modified to suit an individual project, who may wish for additional overrides etc

Note that to override it downstream, you need to set it to a space, not an empty string (or the var sub doesn't work)
i.e. ENV EXTRA_DUMP_CMD=" " in the cli.Dockerfile

I'll push that var into the postgres example to get the tests passing