wasm: envoy specific ABI to add tags to active span

*Title*: *envoy specific ABI to add tags to active span*

*Description*:
Currently, we don't have a way to add tags to the active span for a request being processed by wasm filter. We would like to add an envoy specific ABI which would allow us to do the same.

```
extern "C" WasmResult envoy_set_active_span_tag(const char* key_ptr, size_t key_size,
const char* value_ptr, size_t value_size);
```