adding development time static files proxying

[Erwinvandervalk]

What issue does this PR address?
Right now, the BFF allows you to proxy an index html document. This works great in production, but not so great during development.

Suggestion:
Add a proxying method that proxies all static assets, including web sockets. This works great for development. Then provide an easy method for switching.

fixes https://github.com/DuendeSoftware/products-private/issues/2265

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[Copilot]

Pull Request Overview

This pull request adds support for static file proxying in development environments while maintaining CDN support for production. The main change introduces a new StaticAssetsUrl property to BffFrontend that allows proxying all static assets (JS, CSS, images) through the BFF server, which is particularly useful for development scenarios with hot-reloading dev servers.

• Replaces IndexHtmlUrl with CdnIndexHtmlUrl and adds StaticAssetsUrl for full asset proxying
• Implements mutual exclusion between CDN and proxying modes to prevent configuration conflicts
• Adds extension methods to dynamically choose between CDN and proxying based on environment

Reviewed Changes

Copilot reviewed 26 out of 27 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
BffFrontend.cs	Core model changes adding StaticAssetsUrl and CdnIndexHtmlUrl with mutual exclusion logic
StaticFilesHttpClient.cs	New implementation replacing IndexHtmlHttpClient with static asset proxying capabilities
ProxyStaticFilesMiddleware.cs	Updated middleware to handle both index HTML and static asset proxying
BffFrontendExtensions.cs	New extension methods for configuring static assets with environment-based switching
Test files	Updated tests to use new property names and test static asset proxying functionality
Host projects	Example configurations showing development vs production usage patterns

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[Copilot]

Pull Request Overview

Copilot reviewed 29 out of 30 changed files in this pull request and generated 3 comments.

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