Upgrade retries should be configurable

## What is the problem your feature solves, or the need it fulfills?

Currently, an upgrade will only be attempted for 5 seconds, at a 1/s interval. This is a little annoying when doing the upgrade testing manually:

https://github.com/cloudflare/pingora/blob/42a847d372f1b21334b43c3be03655b12a8cc21a/pingora-core/src/server/transfer_fd/mod.rs#L185-L188

## Describe the solution you'd like

It would be nice if the duration or interval were configurable through ServerConf or another API

## Describe alternatives you've considered

I could just go faster, and this likely won't be an issue when doing automatically. This is just a minor Quality of Life request.

## Additional context

N/A

Start in pingora-core/src/server/transfer_fd/mod.rs at the retry logic around lines 185–188, then inspect ServerConf and related server APIs to determine how configuration is exposed. Done means the upgrade retry duration or interval can be configured through the chosen API instead of remaining fixed at five seconds and one attempt per second.

Written by the indexing model from the issue text.