Mysql default port

[sendmars]

Description of your changes

When connection secret doesn't contain port, this change assumes default port for MySQL, 3306.

This is needed because provider-aws doesn't write port (by default) to the connection secret for Aurora DB cluster. Provider-sql shouldn't accept the empty string as it is an invalid value for port, and assume that a missing value should default to the standard port.

Fixes #76

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

[Testing incomplete] I ran make run locally, which caused a local reconciler to run in parallel with the one in the kubernetes cluster.

When describing the user.mysql on the kubernetes cluster, I see (messages only, for readability):

cannot select user: dial tcp 10.0.x.x:3306: i/o timeout         # local, patched reconciler
cannot select user: dial tcp 10.0.x.x:0: i/o timeout            # reconciler in the cluster

The change sets the port correctly. The connection to db_host:3306 is open, so I would expect that the user is created. The problem seem not to be related to my change, but I would need some assistance in figuring this out. The debug output contains (newlines added manually):

2022-04-05T23:46:44.471+0100    DEBUG   provider-sql    Cannot observe external resource
{
"controller": "managed/user.mysql.sql.crossplane.io",
"request": "/cluster-test-user",
"uid": "...",
"version": "...",
"external-name": "cluster-test-user",
"error": "cannot select user: dial tcp 10.0.x.x:3306: i/o timeout",
"errorVerbose": "dial tcp 10.0.x.2x:3306: i/o timeout
      cannot select user
      github.com/crossplane-contrib/provider-sql/pkg/controller/mysql/user.(*external).Observe
          /workdir/opensource/provider-sql/pkg/controller/mysql/user/reconciler.go:216 ..."

I checked that the port 3306 is part of c.db in Observe. I suspect that the issue with connection comes from the interaction of the two reconcilers.