ENH: Add NiftiJSONExtension class, use for NIfTI-MRS

[effigies]

We haven't kept up with all the NIfTI extensions added over the years. With NIfTI-MRS being on the verge of adoption into BIDS, it seems like the least we can do to support JSON extensions.

I'm not sure if any of the other extensions are JSON, so I'm leaving them as bytes for now. I'm a bit inclined to abstract this out so that we can properly type these things, but we'll see how much I feel the need to procrastinate on more important things.

cc @wtclarke @markmikkelsen

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.20%. Comparing base (e6ccec4) to head (9afa0be). Report is 11 commits behind head on master.

Files	Patch %	Lines
nibabel/nifti1.py	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1327 +/- ## ======================================= Coverage 92.19% 92.20% ======================================= Files 98 98 Lines 12397 12404 +7 Branches 2557 2556 -1 ======================================= + Hits 11430 11437 +7 Misses 644 644 Partials 323 323 ```

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[wtclarke]

Thanks for implementing this. Anything you need from our end?

[effigies]

Not really, just a heads up that this would break any code that expects img.header.extensions[0].get_content() to be bytes instead of a dict. To support multiple versions of nibabel, you could have a little function like:

def get_mrs_header(img: nb.Nifti1Image) -> dict | None:
    exts = img.header.extensions.get_codes()
    if 44 not in exts:
        return None
    mrs_header = img.header.extensions[exts.index(44)].get_content()
    if isinstance(mrs_header, bytes):  # nibabel < 6
        return json.loads(mrs_header.decode())
    return mrs_header

[effigies]

I think I'm leaning toward #1336 instead of this. In this case, you could continue to use json.loads(ext.get_content()) for older nibabel, or you could update to using ext.json() instead once that patch is included in the oldest supported version of nibabel.

[effigies]

Superseded by #1336.