p-token: Add delegate support to unwrap_lamports

[febo]

Problem

Currently it is not possible to use a delegate to unwrap_lamports.

Solution

Add delegate support to unwrap_lamports. The delegate implementation works similarly to how it works on transfer:

• delegates can only unwrap up to the amount delegated
• when the delegated amount reaches 0, the delegate is cleared

[joncinque]

Looks great to me! Just a tiny nit, which you can take or leave