[Blazor wasm] Use provided loadBootResource function to override how config is loaded

[romansp]

[Blazor wasm] Use provided loadBootResource function to override how the config is loaded

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Use "loadBootResource" function to override how the config is loaded.

Description

PR updates WebAssemblyConfigLoader.initAsync to use provided loadBootResource function from startOptions to override how the config is loaded in runtime. Allows loading config from a custom source, such as an external CDN. Providing similar behavior to loading of other boot resources.

I couldn't find any existing tests that cover current loadBootResource behavior for other resources. So, I didn't include any new tests. Though I see that JsInitializersTest.cs might be a good place for that? Let me know if these tests are expected and I will work on this too.

Fixes #43700.

[ghost]

Thanks for your PR, @romansp. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[javiercn]

/AzurePipelines run AspNetCore-ci

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