Add json_get and associated methods

[samuelcolvin]

Add:

• json_get(json: str, *keys: str | int) -> JsonUnion - Get a value from a JSON object by it's "path"
• json_get_str(json: str, *keys: str | int) -> str - Get a string value from a JSON object by it's "path"
• json_get_int(json: str, *keys: str | int) -> int - Get an integer value from a JSON object by it's "path"
• json_get_float(json: str, *keys: str | int) -> float - Get a float value from a JSON object by it's "path"
• json_get_bool(json: str, *keys: str | int) -> bool - Get a boolean value from a JSON object by it's "path"
• json_get_json(json: str, *keys: str | int) -> str - Get any value from a JSON object by it's "path", represented as a string

[samuelcolvin]

this is failing with

called `Result::unwrap()` on an `Err` value: Plan("Cannot infer common argument type for comparison operation Union([(0, Field { name: \"null\", data_type: Boolean, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (1, Field { name: \"bool\", data_type: Boolean, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (2, Field { name: \"int\", data_type: Int64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (3, Field { name: \"float\", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (4, Field { name: \"string\", data_type: Utf8, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (5, Field { name: \"array\", data_type: Utf8, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (6, Field { name: \"object\", data_type: Utf8, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} })], Sparse) = Int64")

Unfortunately that error is occurring before before the rewriter is reached, so there's no way to rewrite the logical plan.