Rework to remove Alexa voiceagent dependencies

Changes include:
- The previous somewhat hard-coded Alexa voiceagent configuration in the
  app controller definition has been replaced with a scheme that allows
  instead specifying voiceagent configuration via the configuration file
  /etc/xdg/AGL/voice-high.json, and individual per-voiceagent
  configuration via JSON files in the directory
  /etc/xdg/AGL/voiceagents. See the updated README.md for more details.
- The new file reading code for the above change was facilitated by
  bumping the C++ standard option to 17 (from 11) to allow use of the
  new standard library filesystem classes.
- The configured voiceagents' APIs are now loaded dynamically using the
  afb_api_require_api call enabled by the recently exposed
  platform:apis:auto-ws widget permission. The widget configuration has
  been updated to add the permission, and also remove the previously
  hard-code dependency on the "alexa-voiceagent" API.
- App controller event hooks are created dynamically for the events from
  the configured voiceagents. This is done by calling back into the
  app controller API at the appropriate place during initialization,
  replacing the previous hard-coding of events from "alexa-voiceagent".
  If future use cases require dynamic removal of voiceagents, then this
  scheme will need to be revisited, as the app controller API currently
  has no provision for removing events.
- The IAFBApi interdace and AFBApiImpl implementation classes have been
  extended with additional getApi and requireApi calls to enable the
  above changes in a straightforward manner.

Bug-AGL: SPEC-2898

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Iae8b99a6286174510e88d8eeffd51718db64f2f4

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