Make `RelationalDatabaseHealthContributor` public and generic

[Levyks]

Is your feature request related to a problem? Please describe.

I would like to integrate non-supported databases into Steeltoe`s healthcheck chain. (Oracle)

Describe the solution you'd like

Make Steeltoe.Connectors.RelationalDatabaseHealthContributor class public, with overrideable Id and CommandText properties.

Describe alternatives you've considered

• Copying the entirety of that class (as well as the needed Steeltoe.Common.Extensions.ExceptionExtensions) to my own project (severe code duplication)
• Implementing my own healthcheck logic (I mean, this works, but Steeltoe's class is perfectly reusable and can be made generic by just changing a few lines)