Make payer optional in ExtendLookupTable instruction

[ckamm]

Problem

Programs that maintain address lookup tables may want to fund these tables fully upfront to avoid passing in a payer and system_program to the ExtendLookupTable instruction.

This comes up when a program wants to take user lamports only once, during some create-account instruction and wants to later extend an address lookup table without needing access to user lamports again.

It can currently be worked around by manually funding the address lookup table account during creation, and then passing a stub payer to ExtendLookupTable. But a cleaner API would be appreciated.

Proposed Solution

Make accounts 2 ("[SIGNER, WRITE] Account that will fund the table reallocation") and 3 ("System program for CPI.") optional in ExtendLookupTable. Possibly add an option to fully fund the table to CreateLookupTable.

@jstarry

[github-actions]

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