encoding.json Marshallers for some of the well known types

[rohitsaboo]

Right now, the only way to support transformation of the well known types between proto and json is jsonpb and unfortunately, some well known types such as Struct and Values are not yet supported for transforming from json to proto. (Also, while I haven't investigated this, others have reported here that jsonpb is ~6x slower.)

It will be useful to have some marshallers and unmarshalers for some of the simpler, more commonly used well known types so that one can use these with encoding.json.

If the proposal sounds good, I can create changelists for a few of these well known types.

[bcmills]

Arguably all generated message types ought to implement json.Marshaler and json.Unmarshaler (calling jsonpb under the hood). But that's a separate issue; I don't think adding a special-case for WKTs is appropriate.

[cybrcodr]

Support for WKT have been implemented in 18c9bb3

[cybrcodr]

Sorry, I missed out the subtle wording here for referring to encoding/json.Marshaler. I closed this bug as 18c9bb3 added support for WKT in jsonpb.

bcmills@ did file related issue #256 for supporting the general case of using encoding/json.