[browser] More flexible boot config schema

[maraf]

When pulled Blazor SDK down to runtime, we chose the Blazor format of boot.json. It's benefit is a compact layout, but it doesn't give a room for adding metadata. We should design a new schema that won't be excessively chatty, but would provide a room for expansion. Random notes

• Leave assets split into groups by behavior
• Each asset should be an object which would allow to attach mutiple key-value
• Migrate internal assets processing to a unified schema

[dotnet-policy-service]

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