Support using convert/update for extension for msgpack codec.

[gabriel]

The msgpack codec assumes WriteExt/ReadExt so the (time) example at http://ugorji.net/blog/go-codec-primer#using-extensions wouldn't work.
This patch allows you to tell it to use ConvertExt/UpdateExt instead.

[ugorji]

Hi,

This solution doesn't fit in with the design of messagepack extensions, which requires that a tag and the length of the subsequent number of bytes be prefixed in the stream, before the encoding happens.

As you see from your changes, the tag is not written and the number of bytes is not inferred before you attempt encoding the value. This means it is not a true msgpack extension, and will probably fail during decoding even on if using the go library. It will also fail when using other libraries.

This only works where the format specifically designs extension support that way (e.g. cbor).

[gabriel]

Is there a way to hook into the codec to do conversions? (That was the intent of this patch.)

For example I wanted to do something like the TimeExt example (which uses ConvertExt/UpdateExt in the msgpack and cbor codec.) Only msgpack uses the WriteExt/ReadExt.