Add config flow to Transmission Integration

[engrbm87]

Description:

This PR adds config flow to the Transmission Integration. it also allows importing from configuration.yaml.
The user will be able to add/remove the monitored_conditions sensors , the turtle_mode switch and set the update interval using the options menu. The user will need to restart HA after changing the options to add/remove the sensors and switch.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#10293

Example entry for configuration.yaml (if applicable):

transmission:
  host: 192.168.1.1

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

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

• Documentation added/updated in home-assistant.io

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

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.