Filter state: Add missing api to get filterstate format working

[vikaschoudhary16]

Commit Message: Filter state: Add missing api to get filterstate format working
Additional Description:
Consider the case where filter state is being set using the set_filter_state filter:

- name: "envoy.filters.http.set_filter_state"
            typed_config:
              "@type": "type.googleapis.com/envoy.extensions.filters.http.set_filter_state.v3.Config"
              on_request_headers:
              - object_key: "envoy.network.upstream_server_name"
                format_string:
                  text_format_source:
                    inline_string: "%REQ(:AUTHORITY)%"
                shared_with_upstream: "TRANSITIVE"

Now if one try to use the value from the filter state, for example in the tunneling config hostname:

- name: encap
    internal_listener: {}
    filter_chains:
    - filters:
      - name: tcp
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.tcp_proxy.v3.TcpProxy
          stat_prefix: tcp_stats
          cluster: cluster_0
          tunneling_config:
            hostname: "%FILTER_STATE(envoy.network.upstream_server_name:PLAIN)%"

Then filter state formatter fails to find the value because here, it invokes serializeAsString() on the stored filter state object, which is UpstreamServerName instance in this case and it is missing this function.

I think similar change would be required for other well-known name objects as well like Upstream subject alt names and alpn application protocols.

related #30674

[vikaschoudhary16]

ping @kyessenov to validate my understanding

[kyessenov]

Yeah, this makes sense. Not sure why serializeAsString was not implemented.