x/txfees: Make an ante-handler filter for recognizing Gamm arb txs, and charging a flat fee for them

[ValarDragon]

To help combat arb bot fee spam, we should add a mempool filter detecting if a tx is an arb transaction. If so, require a different minimum fee than every other tx.

Theres three steps for this:

• Make a gamm function for detecting if this multiMsg is an on-chain arb cycle
• Make the anteHandler method use this for getting minimum gas price
• Extend the app.toml config to expose this (instead of hardcoding something)

We can add new parameters to the app.toml config, e.g. see this PR: https://github.com/cosmos/cosmos-sdk/pull/10561/files

I don't yet know if its possible to add these from the Osmosis repo, or if we have to do it in the SDK fork.