add very basic (and ugly) first implementation

this offers two HTTP endpoints:
- `/image`: returns a single JPEG image
- `/stream`: returns a `multipart/x-mixed-replace` stream of JPEGs

currently all settings (e.g. resolution) are hardcoded and nothing can
be changed by the user. this is a very far cry from what `esp32-camera`
offers. if you need something productive you might be better off using
the [`CameraWebServer` Arduino example].

areas needing improvement:
- don't use `Vec<u8>` for image but stream it directly to HTTP (but
  still in an HTTP-agnostic way if ever possible)
- allow configuring settings (via HTTP endpoint)
- refactor and move generic things out: in the best case there's a
  generic camera trait available somewhere centrally which is
  implemented in HALs/BSPs and applications don't need to care about
  them.
- add documentation
- everything else which might need a cleanup

currently this is based on a fork of `esp-hal` until [esp-hal#3219] is
merged.

[`CameraWebServer` Arduino example]: https://github.com/espressif/arduino-esp32/tree/master/libraries/ESP32/examples/Camera/CameraWebServer
[esp-hal#3219]: esp-rs/esp-hal#3219

Author: rursprung