Proposal - Add Async PreInitAsync to allow async operations such as configuration I

[damianh]

Issue #, if available:

A proposal to address #702

Description of changes:

• Add virtual member PreInitAsync to AbstractAspNetCoreFunction that allows async operations to happen, such as loading configuration from secrets manager, before the webhost is initialized. (I'm not too fond of the name. I'd prefer just InitAsync() and rename Init() to InitWebHost() but that would be a breaking change...)
• It is only invoked when StartupMode is set FirstRequest which in an asynchronous call stack. It is not possible to call it when StartupMode is set to Constructor as invoking async methods in a constructor is an anti-pattern. (If there is no specific advantage to using Constructor startup mode, could it be argued to remove it? 🤷 )
• Add test to assert PreInitAsync() is called. I also noticed there was no tests / test coverage for StartMode.FirstRequest; this test now covers it.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[normj]

I think having this only being invoked if StartupMode is set to FirstRequest, which isn't the default, is going to cause too much confusion.

Since users are forced to make async calls in an sync call right now if they need to load things async like secret manager should we just move the GetAwaiter().GetResult() to the AbstractAspNetCoreFunction and call the PreInit method at the same location Init is called. In the default case Task.CompletedTask is returned so that won't affect performance.

[damianh]

Ok, will attempt your suggestion.

[damianh]

Added a comment regarding this to original issue #702 (comment). Not really happy with this proposal as is so going to close.