EntityKeyTable

[mavaylon1]

Summary of changes

• In the experimental ExternalResources, added a entity_keys table and removed keys_idx from the entities table.

PR checklist for schema changes

• [x] Update the version string in docs/source/conf.py and common/namespace.yaml to the next version with the suffix "-alpha"
• [x] Add a new section in the release notes for the new version with the date "Upcoming"
• [x] Add release notes for the PR to docs/source/hdmf_common_release_notes.rst and/or docs/source/hdmf_experimental_release_notes.rst

[rly]

Thanks for the PR @mavaylon1 . In the future, please update the versions and release notes like the PR checklist says. I took care of it this time