Add Eager Options Validation: ValidateOnStart API

[maryamariyan]

• Supports Options Validation, getting triggered at startup via a hosted service.
• Allows options validation for named options as well.

Note: Microsoft.Extensions.Options would need a new dependency to Microsoft.Extensions.Hosting.Abstractions because we use IHostedService to implement this feature.

Minor TODO:

• Improve unit tests + code coverage
• Get API approved
• Cleanup comments

Fixes #36391

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Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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Issue Details

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• Supports Options Validation, getting triggered at startup via a hosted service.
• Allows options validation for named options as well.

Note: Microsoft.Extensions.Options would need a new dependency to Microsoft.Extensions.Hosting.Abstractions because we use IHostedService to implement this feature.

Minor TODO:

• Improve unit tests
• Get approval on .NET API review meeting tomorrow on:
  (1) Namespace for API (Was originally approved to go under Microsoft.Extensions.Options.DataAnnotations but that is not needed, since the feature is not tied to Data Annotations so it is best to be part of Microsoft.Extensions.Options).
  (2) Finalize name ValidateOnStart (rather than ValidateOnStartup)

Author:	maryamariyan
Assignees:	-
Labels:	area-Extensions-Options, new-api-needs-documentation
Milestone:	-

[maryamariyan]

The CI failure is unrelated.

@davidfowl @eerhardt The PR should be ready for another review.

[eerhardt]

Looks good. Thanks for driving this through, @maryamariyan.

[davidfowl]

Still want to discuss the home

[maryamariyan]

Still want to discuss the home

OK, marking unresolved the conversation thread above: #47821 (comment)

Here's a summary of the two approaches tried:

• Microsoft.Extensions.Options (would need a reference to Microsoft.Extensions.Hosting.Abstractions to access IHostedService for implementing ValidateOnStart
• Microsoft.Extensions.Hosting (in PR as-is, it already references M.E.Options and M.E.Hosting.Abstractions)