Added the ConfigureDefaults API, as an extension method of IHostBuilder

[IEvangelist]

Added the ConfigureDefaults API, as an extension method on the IHostBuilder interface.

• Add corresponding ref entry
• Encapsulate logic from CreateDefaultBuilder(string[] args) into ConfigureDefaults
• The CreateDefaultBuilder(string[] args) method now calls ConfigureDefaults
• The ConfigureDefaults method had a bit of its conditional checks simplified with C# 9 pattern matching
• Remove unused using statements in Host
• Update <summary> tags accordingly
• Added unit test to verify a default config value is applied

Fixes #36003

[dotnet-issue-labeler]

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.

[ghost]

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Issue Details

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Added the ConfigureDefaults API, as an extension method on the IHostBuilder interface.

• Add corresponding ref entry
• Encapsulate logic from CreateDefaultBuilder(string[] args) into ConfigureDefaults
• The CreateDefaultBuilder(string[] args) method now calls ConfigureDefaults
• Added unit test to verify a default config value is applied

Fixes #36003

Author:	IEvangelist
Assignees:	-
Labels:	area-Extensions-Hosting, new-api-needs-documentation
Milestone:	-

[eerhardt]

Thanks for the contribution! Just a few minor comments.

[eerhardt]

LGTM. Thanks again for the contribution here!

[maryamariyan]

Test failure logged in #48696