Add iSamples metadata constants in a library that third parties can import

[dannymandel]

If we are going to ask partners to adopt our metadata format and write out JSON lines sitemaps, we should have a thing client library that lets them import the constants so they can get it right.

In particular these: https://raw.githubusercontent.com/isamplesorg/isamples_inabox/d6b8d310a13f53b64a24cf1d211d24be28f1e98e/isamples_metadata/metadata_constants.py

But maybe there is other useful things? Particularly the vocabulary mapping stuff -- that seems like a no brainer to share somehow.

[dannymandel]

It probably makes sense to wrap a vocabulary library around the public endpoints:

https://central.isample.xyz/isamples_central/vocabulary/sampled_feature_type
https://central.isample.xyz/isamples_central/vocabulary/material_sample_type
https://central.isample.xyz/isamples_central/vocabulary/material_type