Implement maxAvailableShares

[MickdeGraaf]

User Story

• As an <audience/user>: Integrator
• I want to <action/task>: be able to fetch how much pool tokens can be withdrawn.
• so that <outcome/goal/benefit>: Can show this to end users

Type

• Enhancement
• Maintenance
• Refactor

Description

Returns the maximum amount of pool tokens that can be withdrawn

Definition of Done

• Implement maxAvailableShares
• Unit test maxAvailableShares

Attach files or screenshots if it's helpful for this issue.

Part of #30
See Yearn implementation for reference