Added support to Amcrest camera to feed using RTSP via ffmpeg

[tchellomello]

Description:

Added support to Amcrest camera to feed using RTSP via ffmpeg.
This patch is also an alternative to aiohttp not supporting HTTPDigest authentication.

Of course is expected the ffmpeg to be installed on the system. Documentation will be updated pointing to https://home-assistant.io/components/ffmpeg/ install as pre-requisite.

Related issue (if applicable): fixes #6884 #7398

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2654

Example entry for configuration.yaml (if applicable):

#cameras.yaml
- platform: amcrest
  name: "Amcrest Backyard"
  host: !secret amcrest_backyard_host
  username: !secret amcrest_backyard_username
  password: !secret amcrest_backyard_password
  port: 80
  resolution: low
  stream_source: rtsp

- platform: amcrest
  name: "Amcrest Driveway"
  host: !secret amcrest_driveway_host
  username: !secret amcrest_driveway_username
  password: !secret amcrest_driveway_password
  port: 80
  resolution: high
  stream_source: rtsp

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New dependencies are only imported inside functions that use them ([example][ex-import]).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

Many thanks to @pvizeli on explaining how ha-ffmpeg works.

[mention-bot]

@tchellomello, thanks for your PR! By analyzing the history of the files in this pull request, we identified @andrey-git, @fabaff and @balloob to be potential reviewers.