Add pluggable URL scheme resolver (e.g. blob:) to UrlRequest

[bkaradzic-microsoft]

What

Adds a pluggable URL scheme resolver hook to UrlRequest, so a consumer can register a resolver for a non-transport URL scheme (e.g. blob:) and have every UrlRequest-based consumer (fetch, XMLHttpRequest, image/video src, texture loaders, ...) resolve such URLs uniformly through the transport layer instead of each carrying its own branch.

UrlRequest::RegisterSchemeResolver("blob", [](const std::string& url) -> UrlSchemeResolverResult {
    // look up url in your store...
    return { /*handled*/ true, UrlStatusCode::Ok, "OK", contentType, bodyBytes };
});

How

• New public API: UrlRequest::RegisterSchemeResolver(std::string scheme, UrlSchemeResolver resolver), plus the UrlSchemeResolverResult struct and UrlSchemeResolver alias.
• When a UrlRequest is opened with a URL whose (lower-cased) scheme has a registered resolver, the platform transport is bypassed. Resolution is deferred to SendAsync() (rather than Open()) so a URL revoked between open() and send() is honored.
• A resolver that reports handled == false (e.g. a revoked blob: URL) leaves the status at 0/None and records a transport-style error, mirroring how a genuine network failure surfaces.
• The registry is process-global and mutex-guarded. All logic lives in the shared ImplBase + the shared wrapper, so every platform backend (Win32/Unix/Apple/Android) inherits it with no per-platform changes.

Testing

Builds clean with the Win32 backend (UrlLib and UrlLibTests targets). Downstream, this powers URL.createObjectURL blob: support in JsRuntimeHost — see BabylonJS/JsRuntimeHost#207 (tracked by BabylonJS/JsRuntimeHost#213), where all 216 unit tests pass with fetch/XMLHttpRequest resolving blob: through this hook.

Opened because BabylonJS/UrlLib has Issues disabled; JsRuntimeHost#207 currently pins its UrlLib dependency at this fork commit and should be repointed once this lands.

[Copilot]

Pull request overview

This PR adds a process-global, pluggable URL scheme resolver mechanism to UrlRequest, allowing non-transport schemes (e.g. blob:) to be resolved via a registered callback and served through the existing UrlRequest consumer surface (fetch/XHR/resource loaders) without per-consumer special-casing.

Changes:

• Introduces UrlSchemeResolverResult and UrlSchemeResolver, plus the public UrlRequest::RegisterSchemeResolver() API for process-global registration.
• Diverts UrlRequest::Open() when a registered scheme is detected, and performs resolver execution during SendAsync() to honor revoke-after-open scenarios.
• Adds shared-layer storage for resolver state and exposes resolved buffers via ResponseBuffer() for scheme-resolved requests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Source/UrlRequest_Shared.h	Hooks scheme diversion into Open(), resolves via resolver path in SendAsync(), and routes ResponseBuffer() for resolved responses.
Source/UrlRequest_Base.h	Implements the resolver registry, scheme detection, and shared-layer scheme resolution response population.
Include/UrlLib/UrlLib.h	Adds the public resolver API surface (UrlSchemeResolverResult, UrlSchemeResolver, RegisterSchemeResolver).

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[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

LGTM. A few optional comments inline (one API-design point, two minor nits).