cli: replace the 'vault' cmd with cli group with a 'rpc-serve' subcmd

so that one should use:

swh vault rpc-server

as in other swh packages.

The deprecated swh-vault command should still work as expected.

Also improve config file loading behavior as well as the help for this.

Resulting help msg looks like:

$ swh vault rpc-serve -h
Usage: swh vault rpc-serve [OPTIONS]

  Software Heritage Vault RPC server.

  If the CONFIGFILE option is not set, the default config file
  search will be used; first the SWH_CONFIG_FILENAME environment
  variable will be checked, then the config file will be searched
  in:

  - ~/.config/swh/vault/server(.yml|.ini)

  - ~/.swh/vault/server(.yml|.ini)

  - /etc/softwareheritage/vault/server(.yml|.ini)

Options:
  -C, --config-file CONFIGFILE  Configuration file.
  --no-stdout                   Do NOT output logs on the console
  --host HOST                   Host to run the server
  --port PORT                   Binding port of the server
  --debug / --no-debug          Indicates if the server should run in
                                debug mode
  -h, --help                    Show this message and exit.

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Migrated from D1481 (view on Phabricator)