Allow 'extras' field to be modified on all objects in QCArchive

[dotsdl]

Is your feature request related to a problem? Please describe.

There are cases in which e.g. Molecules have been submitted previously, but now we want them to have additional metadata for downstream use in benchmarking. Modification of a molecule after submission is not allowed (for good reason!), but there are cases where post-hoc modification of descriptive data that doesn't impact computation is of real value.

Describe the solution you'd like

The extras field on an object in the QCArchive ecosystem is explicitly not part of the schema, so modification of it on an object in the database should be permitted, even with caution. This would give a single known point on an object that can mutate with time, enabling downstream work that can also change with time. It is the responsibility of the modifier to ensure their changes are appropriate to the downstream consumers of that extras metadata.

Describe alternatives you've considered

Alternatives to this include generating content downstream that could otherwise be stored persistently in extras. This isn't always easy to do: information useful for downstream work, such as canonical smiles for a molecule, may not be as easy to generate downstream as it is upstream to submission.

Adding extras to an existing Molecule could be done with the same submission artifacts that were used for submission, addressing this problem.

Additional context