Add tBTCv2 pool

[lukasz-zimnoch]

What I did

Added a metapool for tBTC v2.

How I did it

The pool is a minimal modification of the existing tBTC v1 pool. It uses sBTC as a base.

How to verify it

Confirm that the tests are passing within the CI.

[iamdefinitelyahuman]

Thank you for your submission! This looks like it'd be easily handled using our new pool factory - https://curve.fi/factory