Provide a pcli interface to post xyk approximations

[erwanor]

As part of #1908, we would like to have a pcli command that generates a collection of Positions approximating a specified payoff. This should be broken into two commands:

• Approximating a CPMM without posting the Posititons:

pcli view dex approximate constant-product <PAIR> <QUANTITY>

• Approximating and posting the orders:

pcli tx position approximate constant-product <PAIR> <QUANTITY>

In the future, it would be nice to be able to segment approximated positions from the rest of the open orders so that one can perform group actions on these positions e.g. "cancel all", "display" etc.

[conorsch]

Unlikely to ship for 51, given higher priority on IBC & DEX.