Simplify health check code in .NET Aspire hosting integrations.

[mitchdenny]

At the moment most hosting integrations are wired up to listen for the ConnectionStringAvailableEvent to capture the connection string and then stuff that value into a captured variable which is shared with the health check registration code. This code is overly cumbersome and we would like to improve it.

The main issue is that typically most of the callbacks to configure clients on the health check registration methods are synchronous which means we can't get the connection string without doing sync over async.

We've filed this issue on the repo containing a lot of the health checks we are using:

Xabaril/AspNetCore.Diagnostics.HealthChecks#2306

That links to this PR which shows an example of the changes we are after applied to Redis:

Xabaril/AspNetCore.Diagnostics.HealthChecks#2305