[Blazor] Custom ICU loading from runtime path #90018

First merge #89771, then fix this one.

• Load properly a user-created custom file from a full path - easy to fix.
• Load a runtime-pack custom file from a full path:

/workspaces/runtime/artifacts/bin/dotnet-latest/sdk/8.0.100-rc.1.23404.1/Sdks/Microsoft.NET.Sdk.StaticWebAssets/targets/Microsoft.NET.Sdk.StaticWebAssets.targets(475,5): 
error : Conflicting assets with the same target path '_framework/icudt.dat'.

Even when we make sure that icudt.dat filename is removed from the @(WasmNativeAsset) item before adding it, it still keeps a duplicated entry with this value. Most probably some Task is adding it after Blazor SDK's _ResolveGlobalizationConfiguration is executed but no idea how.

• Load a runtime-pack custom file from a relative path:
  In Wasm app we are able to load the files from paths relative to runtime pack because we have access to $(MicrosoftNetCoreAppRuntimePackRidNativeDir) value and we append it as a prefix in case the path passed to custom icu msbuild property is not absolute:
  

  runtime/src/mono/wasm/build/WasmApp.targets

  Line 364 in d2e6efb

  <_WasmIcuDataFileName Condition="'$(HybridGlobalization)' != 'true' and '$(WasmIcuDataFileName)' != '' and !Exists('$(WasmIcuDataFileName)')">$(MicrosoftNetCoreAppRuntimePackRidNativeDir)$(WasmIcuDataFileName)</_WasmIcuDataFileName>

  
  In Blazor we don't have access to that directly. We could extract it assuming that e.g. dotnet.js is in the runtime pack location and assuming that its added to @(WasmNativeAsset) but it's not a perfect solution.
  cc @lewing