Batch-check sponsor wallets for funding

[bbenligiray]

Airseeker checks the sponsor wallets specified in its config file at the start and discards the ones that are not funded api3dao/airseeker#309 This is especially useful for self-funded, Nodary-style usage where an Airseeker has hundreds and thousands of sponsor wallets.

This can be further improved by multi-calling the balanceOf() calls to the sponsor wallets, resulting in fewer RPC calls. Api3ServerV1 inherits ExtendedSelfMulticall, so the multicall can be done to that contract.