feat(Publisher.ApplicationInsights): add connection string support

[skyfrk]

What this PR does / why we need it:

On March 21, 2025 Application Insights will deprecate instrumentation key based data ingestion (azure-deprecation/dashboard#212).

AspNetcore.HealthChecks.Publisher.ApplicationInsights supports instrumentationKey as configuration option for TelemetryClient.
Microsoft suggests to migrate to connection string-based authentication instead.

Which issue(s) this PR fixes:

azure-deprecation/dashboard#212

Special notes for your reviewer:

Please add the label hacktoberfest-accepted to this pull request, so that this pull requests counts as contribution towards https://hacktoberfest.com .

Does this PR introduce a user-facing change?:

Yes, it introduces a new optional parameter connectionString in the signature of the method AddApplicationInsightsPublisher(...).

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 <-- not needed, I guess
• Provided sample for the feature

Sample with connection string:

public void ConfigureServices(IServiceCollection services)
{
    services
        .AddHealthChecks()
        .AddApplicationInsightsPublisher(connectionString: "InstrumentationKey=00000000-0000-0000-0000-000000000000;IngestionEndpoint=https://custom.com:111/;LiveEndpoint=https://custom.com:222/;ProfilerEndpoint=https://custom.com:333/;SnapshotEndpoint=https://custom.com:444/;");
}

[sungam3r]

This change is breaking. Both at binary level and source level - you should point compiler what arg to pass - instrumentationKey: or connectionString:. For someone it can be a problem though I don't see a way to do better decision.

[skyfrk]

So, can we merge this PR?

[sungam3r]

Maybe after migrating repo to .NET 7 SDK and bump major version? @carlosrecuero ?

[sungam3r]

#1533

[sungam3r]

@skyfrk please pull changes from master to verify CI. Also now is a good time to go on with any breaking changes if any. See #1574.

[skyfrk]

I synced my fork :)

[sungam3r]

Thanks. Waiting for CI.

[sungam3r]

I added label.

[sungam3r]

Will be released as a part of #1593 .