Expose liquidity position management in pcli

[zbuc]

We don't expect that in the long term, pcli will be used much for position management, but we'll use it as an MVP to exercise the chain logic at first. We need:

• Planner support for PositionClose
• Planner support for PositionWithdraw
• pcli tx position close POSITION_ID
• pcli tx position withdraw POSITION_ID
• Minimal smoke test support for position management #1909

[hdevalence]

Folding this into #1724