wasm: Add support for Dynamic Metadata

Description: Currently in WASM is not possible to set Dynamic Metadata. This metadata emitted by a filter can be consumed by other filters and useful features can be built by stacking such filters For example, a logging filter can consume dynamic metadata from an RBAC filter

One of the use cases is the integration with RBAC for example how MYSQL does We (vmware platform team) have a WASM network filter and we'd need to integrate it with RBAC.

Do others find this valuable? Is there something that we should know before we start contributing this? ( cc @venilnoronha, I spoke with him about that)

ref: Rust SDK Issue: https://github.com/proxy-wasm/proxy-wasm-rust-sdk/issues/81

More details:

I did a test by changing the envoy proxy wasm context.cc ONLY for test ( thanks to @yuval-k for pointed there)

WasmResult Context::setProperty(absl::string_view path, absl::string_view value) {
  auto* stream_info = getRequestStreamInfo();
  if (!stream_info) {
    return WasmResult::NotFound;
  }
  // ENVOY_LOG(debug, "***** setProperty wasm path:{}  value:{}", path, value);
  ProtobufWkt::Struct metadata_val;
  auto& fields_a = *metadata_val.mutable_fields();
  std::string v1;
  absl::StrAppend(&v1, value);
  fields_a[v1].set_bool_value(true);
  std::string key3;
  absl::StrAppend(&key3, path);
  stream_info->setDynamicMetadata(key3, metadata_val);

Rust code:

self.set_property(vec!["my.filters.network.network"],
                              Some("myitem".as_ref()));

And actually the filter works:

[2021-02-20 16:45:17.749][7215663][debug][rbac] [source/extensions/filters/network/rbac/rbac_filter.cc:45] checking connection: requestedServerName: , sourceIP: 127.0.0.1:54608, directRemoteIP: 127.0.0.1:54608,remoteIP: 127.0.0.1:54608, localAddress: 127.0.0.1:5673, ssl: none, 
dynamicMetadata: filter_metadata {
  key: "my.filters.network.network"
  value {
    fields {
      key: "myitem"
      value {
        bool_value: true
      }
    }
  }
}

The RBAC seems to work:

[2021-02-19 20:02:20.480][6826232][debug][rbac] [source/extensions/filters/network/rbac/rbac_filter.cc:125]
enforced denied, matched policy myitem-policy

envoy conf:

filter_chains:
    - filters:
        - name: envoy.filters.network.wasm
          typed_config:
            "@type": type.googleapis.com/envoy.extensions.filters.network.wasm.v3.Wasm
            config:
              configuration: {"@type":"type.googleapis.com/google.protobuf.StringValue",
                              "value":""}
              name: "my.filters.network.network"
              root_id: "my.filters.network.network"
              vm_config:
                vm_id: "my.filters.network.network"
                runtime: envoy.wasm.runtime.v8
                code: {"local":{"filename":"wasm32-unknown-unknown/release/my_network_filter.wasm"}}
                allow_precompiled: true
        - name: envoy.filters.network.rbac
          typed_config:
            "@type": type.googleapis.com/envoy.extensions.filters.network.rbac.v3.RBAC
            stat_prefix: rbac
            rules:
              action: DENY
              policies:
                "myqueue-policy":
                  permissions:
                    - metadata:
                        filter: my.filters.network.network
                        path:
                          - key: myitem
                        value:
                           bool_match: true

Note: My RBAC filter does not work always, I am looking why.