Create a payload generator for AppChain message types

[mkysel]

Similar to https://github.com/xmtp/xmtpd/pull/1111

The payer portal might need a way to generate paid traffic.

We need something that takes a private key and sends valid payloads to the blockchain and pays for itself.

[mkysel]

@fbac @api-Hypernova do we need this at this time? Close or backlog?

[mkysel]

@lauchness do you need us to generate traffic for the L3 appchain message types? (so there is some gas consumption) for your current payer portal work?

[api-Hypernova]

Hey @mkysel, do we still need something here?

[mkysel]

This might become very useful if we want to start isolating the chain perf issues.

[api-Hypernova]

Will add something in here: https://github.com/xmtplabs/d14n-node-loadtest Need to support all operation types with a tunable percentage.