Add MySql support to the Certificate DB

[rblackin]

Adding support for the Certificate tables to be used on MySql.

[jstedfast]

This looks fine to me, but w/o unit tests I can't verify it.

You'll note that MimeKit.csproj has the following:

<Compile Remove="Cryptography\NpgsqlCertificateDatabase.cs" />

The reason for this, is again, the inability to write unit tests for PostgresSQL.

I would be happy to accept this PR, but I will likely need to remove it from the default build in the same way that Npgsql is being removed.

[jstedfast]

@rblackin any thoughts/preferences? Also, if you know of a good way to add unit tests that would be able to setup/connect to/make use of a MySQL database, I'd love to know because if that's something that you might know how to get working, then it would be the ideal situation because I would actually love to include this.

[jstedfast]

Helpful hint to myself later: maybe this guide would be useful? https://medium.com/@michaelmatere606/configuring-mysql-for-github-actions-a-step-by-step-guide-f49e0a7aeedc