Added serde derived Serializer behind a feature flag

[josephhajduk]

Did not add it to builders or error types.

This was useful for me, thanks for the very useful library. I made a similar PR to sdp-types

[josephhajduk]

I didn't need Deserialize but it is easy enough to add it, I am using it for tracing/logging.

[sdroege]

I see, that makes sense

[sdroege]

  namespaced features with the `dep:` prefix are only allowed on the nightly channel and requires the `-Z namespaced-features` flag on the command-line

That's OK, let's update the MSRV to something more reasonable that supports this :) I'll do that, don't worry about it.

[josephhajduk]

Great thanks!

[sdroege]

Did not add it to builders or error types.

OOC, why did you add it to error types in sdp-types but not here?

[josephhajduk]

I was wildly pasting in sdp-types and didn't think it through, I don't think it makes sense either I will remove it