Support Health Checks in MySqlDataSourceBuilder

[bgrainger]

There is an existing MySQL health checks package: https://www.nuget.org/packages/AspNetCore.HealthChecks.MySql

It is just initialized from a connection string, so it doesn't integrate with MySqlDataSource, use the logging that's configured, etc.

Design a mechanism to support health checks (perhaps an extension method on MySqlDataSourceBuilder, perhaps in MySqlConnector.DependencyInjection or perhaps in a new package) that integrates easily with a MySqlDataSource.

[bgrainger]

Support for MySqlDataSource has been added here: Xabaril/AspNetCore.Diagnostics.HealthChecks#2096.

If that PR is merged, we should endorse the community-created health checks library instead of creating our own version.

[bgrainger]

https://www.nuget.org/packages/AspNetCore.HealthChecks.MySql should be used for ASP.NET Core health checks.