DescribeTable response shape

[jackye1995]

Currently the DescribeTable response is in unfinished state, only have location and a free-form property map.

Ideally, similar to Hive GetTable and Iceberg LoadTable, it should provide the full table metadata.

The challenge in Lance is that:

(1) Lance metadata is in protobuf. So we need to either return a protobuf bytes section, which could be a challenge in API readability and usability, or if we should just provide a full JSON definition (through mechanism like https://github.com/solo-io/protoc-gen-openapi) to be used in OpenAPI, and then convert it back to Protobuf to be used back in Lance.

(2) even in protobuf, our manifest contains both the main Manifest section and the IndexSection. This is another place that we have not thought through.