feat: make json loader read dtypes correctly and also load small json values

[jokasimr]

1. Previously the json chexus reader loaded type instead of dtype which seems to be the correct field to read from the Nexus JSON templates.

2. The json reader also didn't store any datasets, which makes sense because we don't want to run out of memory if we read large files. But that also breaks a lot of useful cases, mainly validating transformation chains.

So here I made it store only small values up to 100KB.
Unless the file has ~80K small datasets we are unlikely to run out of space with that limit.

3. Loading bifrost json files failed before because they contain "link" entries that were not recognized by the json reader. This PR does not process such entries, but it makes them ignored instead of raising errors.