feat(pd): add --gas-price-simple flag to generate

[conorsch]

Describe your changes

Adds a new CLI flag for pd testnet generate, to opt in to non-zero gas
prices. It's a bit of a shortcut to reuse the same numeric value across
all of them, but we can always add more CLI flags later to finetune.

Closes #4154.

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

  Adds opt-in behavior, and modifies CI runs, but does not touch application code.

Testing/review

The simplest way to validate the new behavior here is:

0. check out this feature branch
1. cargo run --bin pd -- testnet generate --gas-price-simple 5
2. start cometbft/pd to serve the local devnet
3. configure pcli to use http://locahost:8080 as grpc_addr
4. run pcli --home <path> view balance and confirm you have multiple denoms
5. run pcli --home <path> tx send --to <different_addr_in_same_wallet> 1test_usd
6. run pcli --home <path> view balance and confirm that 1) the sent denom moved; and 2) a wee bit of penumbra was burned to pay the fee.

[TalDerei]

LGTM!

so the trick here is to use another denom? I'm looking forward to seeing how our modified planner logic fares in this smoke test environment.

[conorsch]

so the trick here is to use another denom? I'm looking forward to seeing how our modified planner logic fares in this smoke test environment.

To my surprise, I didn't need to modify the smoke tests at all, because the tx send tests already use a non-staking token TEST_ASSET, so the addition of fees doesn't affect the balance checks of TEST_ASSET.