Add mechanism for host to provide callback for getting assembly data

[elinor-fung]

The runtime should enable a way for the host to provide the data representing an assembly. This is a similar idea to the existing bundle_probe, but with the host fully handling populating the data (as opposed to it being an offset into the current executable that the runtime then maps).

To do this, we can update the host_runtime_contract with a new callback:

// Probe for an assembly by `path`. Sets the data representing the assembly (`data`, `size`)
// Returns true if found, false otherwise.
bool(HOST_CONTRACT_CALLTYPE* external_assembly_probe)(
    const char* path,
    /*out*/ void** data
    /*out*/ int64_t* size);

This will be called before the default assembly resolution by the runtime. If both bundle_probe and external_assembly_probe are set, bundle_probe is called first.

• Add callback to host-runtime contract and make runtime invoke it as part of assembly resolution
  • [Android] Add callback to host-runtime contract for getting assembly data #112705
• Host tests for the new callback
  • Add test for external assembly probe mechanism #113007
• Refactoring/renaming to separate external assembly probe vs bundle probe
  • Refactor external_assembly_probe to be separate from single-file bundle probing #113356

[dotnet-policy-service]

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
See info in area-owners.md if you want to be subscribed.