Add extended_symbol type

[benjiewheeler]

This PR adds the extended_symbol type

Simply copied from ExtendedAsset and made neccessary changes.

[aaroncox]

Based on the discussions in Telegram that started here:

https://t.me/antelopedevs/334145

This isn't a builtin, so we don't want to add it as a builtin type for the serializer - meaning it still needs to be defined currently in either the ABI or as a Struct.

As somewhat of a compromise to at least make it more accessible, I did add the type in as an exported struct here: 015df9f

Applications can leverage that as a type if need be.

I'm going to close out this PR since it won't be merged - but in the future if we hit a point where the ExtendedSymbol is added as a builtin type to the platform code, we can definitely get it added here for parity's sake.