add flag to set oauth redirect host in dev mode

[anbraten]

This setting allows to set a special redirect host address (like http://localhost:8000) and still use WOODPECKER_HOST for the public address which is for example used for the webhooks.

[anbraten]

Maybe it would also be a solution, to make it the other way around, so you can set the normal host (WOODPECKER_HOST) to http://localhost:8000 and have an additional option WOODPECKER_DEV_WEBHOOK_HOST to set the host for webhooks differently. Need to check where WOODPECKER_HOST is used as well.