Fetching PrivateKey from Environnement and Execute onchain transaction.

[Ethnical]

Description

This PR allows to fetch the private-key from the env directly and remove the hardcoded dummy privatekey from the codebase.
This will make sure that the private key fetch is with the correct format.

Using the environnement variable PSPEXECUTOR_MON_PRIVATE_KEY like below:

export PSPEXECUTOR_MON_PRIVATE_KEY=2a871[....]3dff6d409c # removed the `0x` for better clarity but this one can be kept. 

I also increase the robustness of the HTTP method /api/psp_execution to make sure only the exact amount of args are strictly necessary also added a protection again to large payload size (> 1MB) that was missing (added some tests accordingly).

Tests
I have refactor and added some tests like:

• TestCheckAndReturnPrivateKey(): tests the CheckAndReturnPrivateKey() function to ensure that it returns the correct private key or error for an incorrect private key provided.

I have refactors also some previous tests to follow the current standard with expectErr instead of wants.

PASS
ok      github.com/ethereum-optimism/monitorism/op-defender/psp_executor        0.391s

[raffaele-oplabs]

LGTM