Add Anker support to Solido.js library

[ruuda]

A Typescript library for interacting with Solido is in progress (#467, #478). We should add support for the Anker program to it as well.

Features can be very basic:

• Deposit and withdraw (not highest priority, but we should use this for the staking widget, and then we need it anyway).
• Return the bSOL price in SOL.
• Return the TVL (amount in the stSOL reserve), in SOL value.