[StaticWebAssets] Process endpoint definitions in parallel

[javiercn]

• Do not pass ExistingEndpoints to the DefineStaticWebAssetEndpoints calls for build assets (they aren't needed).
• Avoid deserializing the full endpoint (only the asset file and the route are used to avoid adding duplicate endpoints).
• Process endpoints in parallel

Around ~1s saving

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[MackinnonBuck]

Looks good, just a few small suggestions.