New Feature Request: support multi fallbacks

Multiple fallbacks are supported.

One implementation for reference:
- serverless-registry can be configured with multiple domain names. For example: `docker-mirror.example.com`, `quay-mirror.example.com`
- Config Multiple fallbacks
- Depending on the requested domain name, fallback to the corresponding backend. For example, if the requested domain name is: `docker-mirror.example.com`, then fallback to `docker.io`.

No files, tests, or entry points are named in the issue. Start by locating the fallback configuration and request-routing entry points, then determine how multiple domain-to-backend mappings should be represented and verified. Done means each configured requested domain selects its corresponding fallback backend.

Written by the indexing model from the issue text.