horcrux-proxy should allow filtering sentries by namespace

[nourspace]

Like the -a/-all flag, we could pass -n/--namespace flag with one, more, or a regex to match namespaces. Only pods in the matched namespaces will be connected to.

The motivation is to allow running multiple proxy instances each responsible for connecting certain sentries with a dedicated horcrux instance. Useful for mainnet/testnet separation, also for testing newer versions of horcrux in isolation.