- make NpgSqlHealthCheckOptions.DataSource internal, so the users can't make mistakes by assigning it to a new data source for every health check invocation
- make sure not more than a single DataSource is created
- when the user provides ConnectionString, try to resolve DataSource just once. If it's present and the connection string is the same, reuse it

- offer two public NpgSqlHealthCheckOptions ctors: one that requires ConnectionString and another that requires DataSource. This allows us to ensure that every instance of this type is valid.
- make DataSource property public, but readonly. This allows us to ensure that the customers are not providing both.

…nd why

…ly a connection string was provided