Add linter checks to GHA & Fix code to pass lints.
The linters added are:
- whitespace: Tool for detection of leading and trailing whitespace
- gosec: Inspects source code for security problems
- gci: Gci control golang package import order and make it always deterministic
- misspell: Finds commonly misspelled English words in comments
- gomnd: An analyzer to detect magic numbers

The file utils/utils.go is excluded from the checks of gomnd, as uses magic numbers through the code

Implements the hash to be signed from PoolL2Tx, compatible with javascript version.
Implements the signature verification for a given PoolL2Tx.

Co-authored-by: Eduard S <eduard@iden3.io>