improvement: Server.WithHealth supports multiple calls by appending

[bmoylan]

Previously, all user-provided healthchecks must be provided in a single call to WithHealth. This has forced applications to build their own healthcheck accumulators.

I have written this code in multiple repos:

server.New().
WithInitFunc(func(ctx context.Context, info witchcraft.InitInfo) (cleanup func(), rErr error) {
	var healthchecks []status.HealthCheckSource
	registerHealth := func(s status.HealthCheckSource) { healthchecks = append(healthchecks, s) }
	defer info.Router.WithHealth(healthchecks...)

	// use registerHealth as a variable to pass to other constructors or things that produce a healthcheck
}

This behavior change is technically a break if anyone is relying on overwriting the values, but I have not been able to find and example of that use case. I considered adding a new API for this (AppendHealth?) but decided the API bloat wasn't worth it. Happy to reconsider if others disagree.

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This change is