Check for files on startup when the zilla.yaml specifies paths to files or directories

[vordimous]

Is your feature request related to a problem? Please describe.
If a .proto file is not accessible by Zilla there isn't a clear indication. The current error message is cryptic:

Jun 29, 2023 1:28:14 PM org.eclipse.yasson.internal.Unmarshaller deserializeItem
SEVERE: Problem adapting object of type class io.aklivity.zilla.runtime.engine.config.NamespaceConfig to interface jakarta.json.JsonObject in class class io.aklivity.zilla.runtime.engine.internal.config.NamespaceAdapter
Configuration parsing error: Problem adapting object of type class io.aklivity.zilla.runtime.engine.config.NamespaceConfig to interface jakarta.json.JsonObject in class class io.aklivity.zilla.runtime.engine.internal.config.NamespaceAdapter

Describe the solution you'd like
For elements that map to the file system, it would be nice for zilla to warn or error when it doesn't find the file or folder it is looking for.

Describe alternatives you've considered
The filesystem server and proxy also let you specify files that doesn't exist but don't give you any indication other than an eventual 404, which can be a number of different problems.

Additional context
grpc (protobuf), openapi+asyncapi+openapi-asyncapi, tls (filesystem vault)

[jfallows]

When addressing this we need to take into consideration that the .proto resource(s) could be delivered remotely via http not necessarily local via filesystem.

[vordimous]

Adding some of the filesystem situations I tested.

  http_filesystem_proxy:
    type: http-filesystem
    kind: proxy
    routes:
      - when:
          - path: /
        with:
          path: index.html # if this is excluded, zilla doesn't throw an error
        exit: filesystem_server
  filesystem_server:
    type: filesystem
    kind: server
    options:
      location: /var/www/ # if this is a specific file instead of a dir, zilla doesn't throw an error

[jfallows]

Note: implementing #908 and removing direct files support from grpc binding aligns with the other bindings and makes this bug obsolete, assuming the new filesystem catalog gracefully communicates when local files are not found.

[jfallows]

#908 implemented.