Add changeDelegation to ILockingVault

[DannyDelott]

Based on previous discussion, this PR adds the changeDelegation method to the ILockingVault interface so that we can safely expect all locking vaults to include delegation via this prescribed method name + args.

This was done by copying the changeDelegation signature and docblock from here: https://github.com/element-fi/council/blob/main/contracts/vaults/LockingVault.sol

[coveralls]

Pull Request Test Coverage Report for Build 3314381926

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.126%

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Totals
Change from base Build 2073398413:	0.0%
Covered Lines:	506
Relevant Lines:	523

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💛 - Coveralls