Customizable Up check for Indexer Proxies

Is there an existing issue for this?

• I have searched the existing open and closed issues

Is your feature request related to a problem? Please describe

All Indexer Proxies are currently tested to see if they're working by trying to reach https://prowlarr.servarr.com/v1/ping, but that assumes:

1. The proxy has a path to clearnet
2. The proxy doesn't run afoul of the servarr.com abuse prevention detection

Not all proxies (e.g. I2P) expect to have stable clearnet access, so currently they're largely unusable with Prowlarr.

The servarr.com site has pretty strong abuse prevention filters which make it highly likely connections will be blocked by it that wouldn't be blocked by most other sites. For example, most VPN connections to it are blocked but are accepted by most of the indexers. Tor exit nodes also seem to be geoblocked so they have inconsistent results.

Describe the solution you'd like

An advanced option in the Indexer Proxy configuration that allows specifying a custom endpoint instead of the default https://prowlarr.servarr.com/v1/ping.

For non-clearnet proxies, this would allow configuring an endpoint within the closed network.

It would also allow the servarr.com abuse prevention mechanisms to be independently managed relative to the indexer proxy support, instead of like now where servarr.com abuse prevention directly controls the unrelated prowlarr functionality.

Describe alternatives you've considered

Alternative hardcoded endpoints available from a drop down that have different levels of abuse prevention filtering.
This doesn't solve the clearnet access issue, but it would make it clear if your proxy might be blocked when accessing an indexer due to proxy reputation or other abuse prevention mechanisms.

Anything else?

None