Allow the read/write of JSON-valued Zarr attributes.

[DennisHeimbigner]

A number of other packages that read/write Zarr insert
attributes whose value is a dictionary containing specialized
information. An example is the GDAL Driver convention (see
https://gdal.org/drivers/raster/zarr.html).

In order to handle such attributes, this PR enforces a special
convention. It applies to both pure Zarr an NCZarr format as
written by the netdf-c library.

The convention is as follows:

Reading

Suppose an attribute is read from .zattrs and it has a JSON
value that is a a dictionary. In this case, the JSON dictionary
is converted to a string value. It then appears in the netcdf-c
API as if it is a character valued attribute of the same name,
and whose value is the "stringified" dictionary.

Writing

Suppose an attribute is of type character and its value looks like
a JSON dictionary. In this case, it is parsed to JSON
and written as the value of the attribute in the NCZarr file.
Here the value is the concatenation of all the characters
in the attributes netcdf-c value.
The term "looks like" means that the value's first character is
"{", its last value is "}", and it can be successfully parsed
by a JSON parser.

A test case, nczarr_test/run_jsonconventions.sh was also added.

Misc. Unrelated Changes

1. Fix an error in nc_test4/tst_broken_files.c
2. Modify the internal JSON parser API.
3. Modify the nczarr_test/zisjson program is modified to support
   this convention.

[edwardhartnett]

@WardF is hopefully waiting for the fixes for the filter installation before doing the release. Ward, I assume you will also be merging this and Dennis' other open PR before merging the filter install changes he is going to put up in a new PR?