feat: support proxy-provider priority

[cringewarm]

Use case: two proxy providers. The primary is expensive with great quality but occasionally under maintenance; the secondary is cheap with average quality and is only a backup. I want the selector to prefer the higher-priority provider unless all of its nodes are unavailable, then fall back to lower-priority providers.

This commit implements that behavior. A new priority field on proxy-provider controls the selection order for url-test proxy groups: nodes from higher-priority providers are chosen first; if none are available, selection proceeds to lower priorities.