MINOR: filters: Add flt_has_filter() and use it in proxy_finalize()

MINOR: filters: Add flt_has_filter() and use it in proxy_finalize()

Add flt_has_filter() returning whether a proxy has at least one filter,
from the legacy configurations or from the flattened instances. Use it
in proxy_finalize() instead of testing px->filter_configs directly, so
the filter analyzers are also enabled for proxies using the filter
instances.

proxy_finalize() runs after the filter instances are flattened, so the
helper can safely inspect px->filter_instances.