Support RDS IAM authentication for MySQL

[fortra-cloudops-platform]

Description of your changes

Fixes #106, provides the ability to create MySQL users with the AWSAuthenticationPlugin

I have:

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

How has this code been tested

Honestly I have had issues testing as I don't know how to get the provider to run my image successfully, any help appreciated.

[mcanevet]

I was about to rebase #133, but it's a bit complicated because a lot of things have changed since that PR have been submitted.
So I tested that one and it works fine. Although, the default is mysql_native_password but this value throws errAuthPluginNotSupported error. Except form that it works fine.

[cten]

I found that setting the default to mysql_native_password fails, documentation makes me believe it should work but failed on the MySQL version I tried. I can try to replicate the changes from #133 on current master. I think those might be cleaner code. Only difference would be not setting a default plugin name.

[mcanevet]

At least I can confirm that this PR works fine. I'm using it right now.
Thanks a lot for this work!

[pierluigilenoci]

@chlunde @Duologic, could someone from you review the PR?

[chlunde]

default:"mysql_native_password"

is not handled here?

[chlunde]

since the code only accepts specific values here, I think you should use an enum to limit to the valid types. Alternatively, let the code handle any value, if that possible

[chlunde]

I'm not a maintainer of this project (and not a MySQL user), but as you pinged me I left some comments :)

A couple more:

1. this code needs unit tests so we cover the new branches

2. It might be cleaner if you keep the query building part inside executeCreateUserQuery instead of partially building it outside, because now we get SQL quoting/escaping in different layers of the code

[pierluigilenoci]

@fortra-cloudops-platform, can you check/react to PR comments?