Exposed RelationalDatabaseHealthContributor

[Levyks]

Description

Makes Steeltoe.Connectors.RelationalDatabaseHealthContributor public and adds a way to override the Id property and CommandText used to check the DB (default is "SELECT 1;")

The goal of this is to make it easier to implement custom health checks for non-supported databases (like Oracle).

Fixes #1594

EDIT: another benefit is that this would make it possible to add a supported DB to Steeltoe's healthcheck chain without registering it through Steeltoe's connector itself (connectionString managed manually)

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[Levyks]

@dotnet-policy-service agree