Rename generic-named Extensions containers to more feature-specific names

[Eilon]

This is to avoid having the same namespace+type name in multiple assemblies. (The specific name doesn't matter that much, but something like HealthChecksServiceCollectionExtensions would be fine.)

See:

• https://github.com/aspnet/HealthChecks/blob/dev/src/Microsoft.Extensions.HealthChecks/ServiceCollectionExtensions.cs#L9
• https://github.com/aspnet/HealthChecks/blob/dev/src/Microsoft.AspNetCore.HealthChecks/WebHostBuilderExtension.cs#L9
• https://github.com/aspnet/HealthChecks/blob/dev/src/Microsoft.AspNetCore.HealthChecks/WebHostExtensions.cs#L9