Add endpoint for retriving sample metadata schema

[pkalita-lbl]

The Field Notes app needs to collect sample metadata in a way that's consistent with the main submission portal. The submission portal relies on a JSON-formatted version of the submission-schema which is then provided to a DataHarmonizer instance:

nmdc-server/web/src/views/SubmissionPortal/harmonizerApi.ts

Line 195 in ccd899b

this.schema = (await import('nmdc-submission-schema/project/json/nmdc_submission_schema.json')).default;

. In order to keep the app and the backend in sync we need to not bundle a copy of the schema into the app; instead it should fetch it dynamically from the backend. To support that we should add an API endpoint that returns the schema.