Add message support

[pixelcircuits]

Add support for InputMessage and OutputMessage. As development of the bridge progresses, we will need support for being able to spend and create messages. Refer to the currently implemented spec here, and you can find the parallel rust sdk issue here.

Here is a non exhaustive list of what needs to be done:

• Add InputMessage to inputs.ts in transactions (then also under provider)
• OutputWithdrawal needs to be updated to OutputMessage in outputs.ts (then also under provider)
• Add getMessages to the provider (similar to getCoins)
• Add addMessage and addMessages to transaction-request.ts (similar to addCoin and addCoins)
  • getCoinInputWitnessIndexByOwner should be modified to support the owner field on input messages
  • can still try to auto add a change output but for the base asset

[camsjams]

Will need to remove owner once fuel-core releases a new version from master, see FuelLabs/fuel-core#600