Add request mutation fields to HTTPMirrorPolicy

Expose Envoy's RequestMirrorPolicy.host_rewrite_literal and
request_headers_mutations on VirtualService HTTPMirrorPolicy via two
new fields:

  authority: rewrites the Host (:authority) header on the mirrored
    request. When set, Envoy implicitly disables the default "-shadow"
    suffix that it would otherwise append to the Host header.

  headers: header manipulation rules applied to the mirrored request,
    reusing the existing Headers / HeaderOperations message. Only the
    `request` side is honored; `response` mutations have no effect
    since the mirrored response is discarded.

These let operators retarget mirror traffic at endpoints that perform
vhost-based routing, and inject/strip headers on shadow traffic (e.g.
adding "x-shadow: true" or removing internal-only headers) without
needing an EnvoyFilter.

Note: regenerated networking/v1alpha3/virtual_service.pb.go via
`buf generate` and patched kubernetes/customresourcedefinitions.gen.yaml
manually for the three VirtualService CRD versions. Reviewers should
re-run a full `make gen` to refresh doc and alias artifacts.

Signed-off-by: Adam Buran <aburan28@gmail.com>

Author: aburan28