- rename existing monitoring plugin to make explicitly
  clear that it supports Basic Auth only
- add new monitoring plugin to support the Client Credentials
  OAuth2 flow
- add xoauth2 prototype tool to convert given username and
  token to XOAuth2 formatted string (optionally encoded to
  support SASL XOAUTH)
- update `list-emails` tool to support either of Basic Auth
  or Client Credentials OAuth2 flow depending on which
  config file settings are used
- refresh README to provide coverage for new settings/plugin
  and liberal collection of ref links for context
- refresh `list-emails` config file example coverage
  - update existing file to make clear that it is intended
    for Basic Auth
  - add new file to cover settings for Client Credentials
    OAuth2 flow

refs GH-313