feat: FlashMintExecutor

[pblivin0x]

To Do

• Add unit tests for reactorCallback(), execute(), executeBatch() in test/FlashMintExecutor.t.sol
• Add integration tests for index base deployments

[ckoopmann]

Happy to take over from here.

[ckoopmann]

Note:
In the uniswap x docs I could only find mainnet deployment addresses.
I will do the integration tests on mainnet for now. Should be trivial to extend them to the base leverage tokens afterwards.

Note that i could find arbitrum deployment addresses for the dutch order reactors here

[pblivin0x]

LGTM, are we able to add a linter to CI

[ckoopmann]

LGTM, are we able to add a linter to CI

Added a 'forge fmt' step to the ci, to check the formatting of all solidity code in the repo.
We can review at a later point if we want to get more extensive linting beyond mere formatting changes.