Allow list of available xcache servers to be downloaded

[ponyisi]

Add values.yaml options to allow the specification of a "Virtual Placement Service" site which gives a list of xcache servers to use. In principle this includes also a liveness test, so servers that go down should be removed dynamically from the list we use for transforms. This will be checked if the list hasn't been updated within a certain interval, by default 30 minutes. This new option can fully replace the server list given by cachePrefix.

This is not my favorite way to implement this, but since gunicorn spawns multiple workers which have distinct configuration data, this is the simplest way of ensuring that all app instances will eventually update. It would conceptually be cleaner to store global configuration data that might be updated in a single location (Redis?) but that's a much bigger refactoring.

[Copilot]

Pull Request Overview

This PR adds support for dynamically retrieving xcache server lists from a Virtual Placement Service (VPS) instead of relying on a static configuration. The implementation periodically checks the VPS liveness service to get an updated list of available servers, with configurable check intervals and automatic fallback handling.

• Introduces VPS-based xcache server discovery with liveness checking
• Adds configurable check intervals (default 30 minutes) for server list updates
• Provides Helm chart configuration options for VPS site specification

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
transformer_manager.py	Implements VPS server validation logic and dynamic cache prefix updates
values.yaml	Adds configuration options for VPS site, check interval, and liveness URL
configmap.yaml	Maps VPS configuration values to environment variables

[Copilot]

Imports should be placed at the top of the file rather than inside functions. Move the time and urllib3 imports to the module level.

Suggested change

	import time
	import urllib3