## Description 

Implements `api/list` route that reports all verified sources served.
This info is currently opaque (a client has to know or guess what's
available based on the repos we index), but `api/list` exposes this info
explicitly.

Currently we just report the filename path--in follow up I will update
this to point to the original source URL (like `github.com/...`) but
that requires a bit more lifting.

```json
{
  "mainnet": {
    "0000000000000000000000000000000000000000000000000000000000000001": {
      "address": {
        "path": "address.move"
      },
      "ascii": {
        "path": "ascii.move"
      },
      "bcs": {
        "path": "bcs.move"
      },
      ...
```

## Test Plan 

added test

---
If your changes are not user-facing and not a breaking change, you can
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then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes