chore: add discover_explicit for vault plugin

[parkedwards]

not sure about this one, but this PR adds discover_explicit() to allow a plugin selection that returns a falsey object. specifically for the vault plugin

ran into this behavior that surprised me when i was adding docketeer-slack:

• no DOCKETEER_VAULT set
• ran docketeer start
• crashed with this error about a missing 1P service account

  File "/Users/edward/code-work/docketeer/docketeer/src/docketeer/main.py", line 235, in main
    vault = discover_vault()
  File "/Users/edward/code-work/docketeer/docketeer/src/docketeer/vault.py", line 113, in discover_vault
    return module.create_vault()
           ~~~~~~~~~~~~~~~~~~~^^
  File "/Users/edward/code-work/docketeer/docketeer-1password/src/docketeer_1password/__init__.py", line 6, in create_vault
    token = environment.get_str("OP_SERVICE_ACCOUNT_TOKEN")
  File "/Users/edward/code-work/docketeer/docketeer/src/docketeer/environment.py", line 30, in get_str
    return os.environ[key]
           ~~~~~~~~~~^^^^^
  File "<frozen os>", line 709, in __getitem__
KeyError: 'DOCKETEER_OP_SERVICE_ACCOUNT_TOKEN'

so the surprising behavior was that 1P was selected by default, and if no SA key is provided then the app crashes. this seems right for some single sub-systems (executor? and search maybe). unsure about vault though - id expect this to be unset, especially since it's an optional system. and then any key fetches would fail