Consider use separate fee payer keys for each zetaclient in the Solana E2E test

[ws4charlie]

Describe the Issue
The Solana E2E test is now using one single fee payer private key (the solanaTestKey). To simulate real-world environment, we should considering using separate fee payer keys for zetalients.

Also, try to move the Solana outbound fee payer key generation and pre-funding logic outside the zetaclientd binary (for instance, an external script).

Expected Outcome
A description of what outcome you are seeking.