Support assemblyPath property in host.json

For precompiled functions VS support, we need a way to specify assembly name at the Function App level.

Propose a property `assemblyPath` in `host.json`. Example:
```json
"assemblyPath": "bin\\MyAwesomeAssembly.dll"
```

This assembly can be overridden using the `scriptFile` property in `function.json`.

Start by tracing how host.json and function.json properties are read by the Azure Functions host. Confirm how a host-level assemblyPath should select the assembly and how function.json's scriptFile overrides it; done means both configuration paths behave as described in the issue.

Written by the indexing model from the issue text.