refactor: replace 3 ak.from_json* functions with one

[jpivarski]

Unifying ak.from_json (in-memory), ak.from_json_file, and ak.from_json_schema.

• verify that ak.to_json is up-to-date
• verify that ak_from_json_new was actually finished way back in May
• update tests
• integrate the with-schema case into this function

All of the from_json cases go to RapidJSON for the parsing and are incremental in the JSON parsed (doesn't load a big string into memory, though it may result in a big ak.Array). For line-delimited JSON, a line_delimited=True argument is now necessary. Passing a file name now requires a pathlib.Path, rather than a bare str (which could be JSON text). All file/non-file cases are wrapped as a Python file-like object for reading in RapidJSON, which releases the GIL at all times except when reading from that file-like object.

Technically, the "refactor" commit type might not be correct because this PR is changing the API, but it's more than just adding a "feature" and it's not a "fix".

The new implementations were mostly copied from the following files, now deleted:

• include/awkward/forth/SpecializedJSON.h
• src/libawkward/forth/SpecializedJSON.cpp
• src/awkward/_v2/operations/ak_from_json_schema.py

The last file, in turn, was developed before it as moved; the original location was:

• src/awkward/_v2/operations/convert/ak_from_json_schema.py

[codecov]

Codecov Report

Merging #1617 (bfbc295) into main (9e17f29) will increase coverage by 0.17%.
The diff coverage is 65.66%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/cuda/__init__.py	0.00% <0.00%> (ø)
src/awkward/_v2/_connect/numexpr.py	88.40% <0.00%> (ø)
src/awkward/_v2/_connect/pyarrow.py	88.46% <0.00%> (ø)
src/awkward/_v2/_lookup.py	98.68% <ø> (+1.17%)	⬆️
src/awkward/_v2/numba.py	93.47% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_avro_file.py	66.66% <0.00%> (ø)
src/awkward/_v2/operations/ak_local_index.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_type.py	53.84% <0.00%> (ø)
src/awkward/_v2/_connect/cling.py	24.90% <2.73%> (-1.10%)	⬇️
src/awkward/_v2/operations/ak_transform.py	8.62% <8.62%> (ø)
... and 34 more

[jpivarski]

Finally closes #192!

[jpivarski]

One thing I forgot: it's still missing documentation. (Not this PR.)