Standardize message field naming

[barnjamin]

Across the current messages defined here, we refer to the additional payload as redeemerMessage

https://github.com/wormhole-foundation/example-liquidity-layer/blob/main/universal/rs/messages/src/fast_market_order.rs#L16

https://github.com/wormhole-foundation/example-liquidity-layer/blob/main/universal/rs/messages/src/deposit/fill.rs#L10

However, the CCTP deposit refers to it as payload

https://github.com/wormhole-foundation/wormhole-sdk-rs/blob/6a8c0700db65a2e049a01e5767548f59439dbb3f/universal/raw-vaas/src/payloads/cctp/payloads.rs#L144-L146

We should consider standardizing the name of the field containing extra data to just payload to match others

There may be additional fields that would benefit from standardization as well