Allow modifying Host Header in Ext Proc

> thanks for clarifying
> I think allowing modification of system headers (like :authority) is a seperate feature request which exposes/configures https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/common/mutation_rules/v3/mutation_rules.proto#envoy-v3-api-msg-config-common-mutation-rules-v3-headermutationrules in the ext proc API
>
> we should track this in a seperate issue as its orthogonal to dynamic forward proxy

_Originally posted by @arkodg in [#3560](https://github.com/envoyproxy/gateway/issues/3560#issuecomment-3271523276)_