feat(core): fix subject-mappings in the CLI and add consistent metadata create/update behavior throughout

[jakedoublev]

1. adds subject-mappings CRUD back (with only support at first for creating a Subject Mapping with a known, existing Subject Condition Set Id at least until SCS functionality is added to the CLI as well)
2. adds consistent universal string slice --label flag to Policy Object CREATE flows for metadata creation
3. adds consistent universal string slice --label and a boolean --force-replace-labels to drive update behavior in Policy Object UPDATE flows for metadata update (i.e. policy <policy object> update --label key1=value1 --label key2=value2 --force-replace-labels overwrites all labels with a new map containing just key1 and key2)

Partially addresses: #60
Related to: #72
Closes: #10