Implement transaction builder

[evanlinjin]

Refer to meeting notes: https://hackmd.io/@evanlinjin/bdk_minutes_20221005

[LLFourn]

Expanding on this. Let's try and create a new txbuilder that is split up into stages:

1. Coin control. Grouping inputs together that must be spent together and inputs that cannot be spent together.

• We can achieve cannot be spent together by the user just running the process several times with each group that can be spent together.
• For must be spent together perhaps group them somehow. Each item is something like (Vec<(Plan, TxOut, Outpoint)>.

2. Coin selection.
   Inputs:

• groups of inputs from above
• change policy (how to decide when we're adding change)
• spending plan for drain output (so we know the weight of drain spend).
  output:
• A subset of inputs that satisfies some parameters.
• whether change was added

3. psbt creation. Maps the candidates chosen by coin selection to their plans and uses the plan to create the PSBT input. Maps the drain output from the coin selection (if any) to the drain plan and populates the PSBT with that.

then you sign it!

This depends on #66 and #46

[rajarshimaitra]

Removed myself as assigned as I won't be able to focus on this currently.. Will reassign myself if no takers..