[UI] Add support for custom webhooks HttpClient delegating handlers

[jabadmgm]

What this PR does / why we need it:
Add support to register custom delegating handlers for webhooks HttpClient to mutate requests.

Which issue(s) this PR fixes: #1023

Does this PR introduce a user-facing change?: No

Sample:

services
    .AddRouting() 
    .AddTransient(sp => new CustomDelegatingHandler(tracer))
    .AddHealthChecksUI(setup =>
    {
        setup.AddHealthCheckEndpoint("endpoint1", "http://healthcheckurl");
        setup.AddWebhookNotification("webhook1", "https://webhookurl", "sample payload");
        setup.UseWebHooksEndpointDelegatingHandler<CustomDelegatingHandler>();
    })
   .AddInMemoryStorage();

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[jabadmgm]

@sungam3r hi, I'm not sure how to do end-to-end testing, still trying to figure it out (if you have time to share some guidance I would be grateful)

[sungam3r]

@jabadmgm Changes look understandable though I have never used UI bits of this project.

[jabadmgm]

@sungam3r great, thank you! Do you have an estimate for when this feature will be released?

[sungam3r]

I help to support this project and still have no experience in publishing releases here. ping @carlosrecuero

[jabadmgm]

@carlosrecuero additional question: I'm using v5.0.1, will this feature be available in that version?

[carlosrecuero]

Do you mean v5.0.1 version on HealthChecks or Net (Core)? For new features, we only apply changes into the current version (net6.0). Only some bugs are replicated in previous versions. Sorry for that.

[jabadmgm]

@carlosrecuero my project is using .NET5 and library version 5.0.1 (edit: decided to migrate my service to .NET6!)

[jabadmgm]

hi @alvarodl, any update on when this might be released? 😊