Feat/gauge registry

[skellet0r]

Gauge registry for tracking gauges deployed via the new curve factory. Allows for gauges to auto-register themselves on initialization, given that they are minimal proxies for the gauge implementation which the Factory has in storage.

Features:

• Query the pool address of a gauge
• Query the gauge version
• Query the number of gauges registered
• Query the gauge addresses via index

[skellet0r]

Need to add last_updated getter for cacheing