Resolves CCD-1636

This PR improves the crds_s3_get script by allowing the user to pass in just the filename (mapping or reference file). The script will determine the exact s3 URI based on the observatory and other environment variables. The destination arg is now an optional argument, and defaults to CRDS_PATH env var. It is still possible to pass in the full s3 uri and absolute path for download destination; these changes allow for a much simpler invocation, e.g. "crds_s3_get roman_0088.pmap".

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)