Proto/CLST: get_balance view

What

Adds get_balance view for CLST contract

Why

How

Following the implementation of native views from !19583, replicates the implementation of !19901 (merged) clstz_balance RPC.

Manually testing the MR

Run a sandbox, then:

octez-client --wait none transfer 100 from bootstrap1 to KT1WCsbJx996ebZfutAitHYsZ8FUZFsTdaD7 --dry-run --burn-cap 10
octez-client bake for --minimal-timestamp

then

octez-client run view get_balance on contract KT1WCsbJx996ebZfutAitHYsZ8FUZFsTdaD7 with input "Pair \"tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx\" 0"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR