[chore][encoding/jaegerencodingextension] add marshaler to follow the codec interfaces

[VihasMakwana]

Description: Add marshaler to follow the codec interfaces. We don't know how to marshal in this case so I'm leaving it as "unsupported"

[dmitryax]

Why is this needed? Exporters will be looking at MarshalTraces implementation. If it's not there, it'll be Start time error, which is better than Consume time error

It's ok to have decoding OR encoding extensions.

[atoulme]

Indeed, let's just not implement it.

[atoulme]

It's completely ok if we only support unmarshaling from jaeger. Just that is useful for most components out there.

[VihasMakwana]

@dmitryax @atoulme I thought we'd follow based on this comment

package codec

type Metric interface {
    pmetric.Marshaler
    pmetric.Unmarshaler
}

type Log interface {
    plog.Marshaler
    plog.Unmarshaler
}

type Trace interface {
    ptrace.Marshaler
    ptrace.Unmarshaler
}

It turns out we don't need to, receivers can do the following:

if ext, ok := extensions[componentID]; ok {
  if enc, ok := ext.(Unmarshler); ok {
	  return enc, nil
  }
  ... error
}

and exporters can do the following:

if ext, ok := extensions[componentID]; ok {
  if enc, ok := ext.(Marshler); ok {
	  return enc, nil
  }
  ... error
}

Please let me know if I'm wrong.

Sorry for any inconvenience, I'll be closing this PR as it's not needed. Thanks!