Proxy timeouts should be configurable

[donaldgray]

We currently have a hardcoded timeout value for all Yarp proxy requests, which is set too high (60s across the board).

Make this configurable per target, we may want a default of XXs which targets can optionally override (to avoid needing a value for all). Ideally pull this from config. Can this be read from Yarp config, similar to how we read endpoint addresses?

This is set via ForwarderRequestConfig.ActivityTimeout. This seems to be causing issues if downstream service is in distress - currently only seen with image-server if a general failure happens across all instances. Do we need to consider #769 when implementing this (probably not - this can be timeout and #769 can add circuit breaker or hedging policy for downstream requests?).

[donaldgray]

Closing as done, in release https://github.com/dlcs/protagonist/releases/tag/v1.2.8