feat: drop JSON functions and dependency on substrait-cpp (#100)

Closes #99
Closes #98 

BREAKING CHANGE: drop explicit support for JSON api

Author: benbellick

.gitmodules

@@ -1,6 +1,3 @@
 [submodule "third_party/substrait"]
 	path = third_party/substrait
 	url = https://github.com/substrait-io/substrait
-[submodule "third_party/substrait-cpp"]
-	path = third_party/substrait-cpp
-	url = https://github.com/substrait-io/substrait-cpp

README.md

@@ -136,12 +136,13 @@ relations {
 ```
 
 ## Load a Substrait Plan from JSON
-A substrait plan can be loaded from its JSON representation
-using the ``substrait.json.load_json`` and ``substrait.json.parse_json``
+A substrait plan can be loaded [from the JSON representation
+of the protobuf message](https://protobuf.dev/programming-guides/json/) using the [`protobuf` python library](https://pypi.org/project/protobuf/):
 functions:
 
 ```
->>> import substrait.json
+>>> from substrait.proto import Plan
+>>> from google.protobuf import json_format
 >>> jsontext = """{
 ...   "relations":[
 ...     {
@@ -182,7 +183,7 @@ functions:
 ...     }
 ...   ]
 ... }"""
->>> substrait.json.parse_json(jsontext)
+>>> json_format.Parse(text=jsontext, mesage=Plan())
 relations {
   root {
     input {

src/substrait/json.py

@@ -1,6 +1,6 @@
 from substrait.sql.sql_to_substrait import convert
 import pyarrow
-import substrait.json
+from google.protobuf import json_format
 import tempfile
 import pyarrow.substrait as pa_substrait
 import pytest
@@ -91,7 +91,7 @@ def duckdb_schema_resolver(name: str):
         conn.install_extension("substrait", repository="community")
         conn.load_extension("substrait")
 
-        plan_json = substrait.json.dump_json(plan)
+        plan_json = json_format.MessageToJson(plan)
         sql = f"CALL from_substrait_json('{plan_json}')"
 
         substrait_out = conn.sql(sql)