[strategies] Build incrementally to support BTC staking

[tomjeatt]

We're currently working on the strategies individually. This is how they will be delivered, but we should approach development differently:

1. Understand the end goal of BTC staking
2. Think in terms of utilities/components, which are then used to build strategies, rather than in terms of building a set of discrete strategies.
3. Consider what work is needed in Squid to support this.

Task is to talk through these issues and come up with the best way of approaching them, so that we can work this way when implementing the BTC leveraged long strategy. Outcome should be documented on this ticket.

See also #1509 and #1154