Include runner configuration per task and include global configuration

[sudheesh001]

• Initial attempt at including runner configurations into pa11y-webservices
• This patch include per-task runner configuration with the POST /tasks request including the runners: [] entries
• This patch allows edits of the runner configuration with the PATCH requests with the runners: [] entries.
• The global service can be configured with the runners: [] in config/ as the default.

Firstly thank you to the pa11y community for such an amazing project. In this PR, I am requesting feedback in the attempt of addressing #88. It has been a long time since #127 was proposed and it hasn't had any recent updates on it. I apologize in advance if this might be overstepping on the work done in the previous patch. Enabling this setting was needed and helpful for a project I've been doing recently and I would like to get feedback from the community on if this design is preferable. This would definitely result in changes in the pa11y-dashboard. It'd also be interesting to see if pa11y can expose a pa11y.availableRunners() of some sort.

Signed-off-by: Sudheesh Singanamalla [email protected]