Chore/cleanup linting

[puffitos]

Motivation

General cleanup of repository, adding linting for the go code and automatic testing (no automatic E2E) before committing, to assure better code quality and non-broken commits.

This is in preparation for the new feature, to allow the COSIGN_REPOSITORY variable to be used by user workloads.

Changes

• E2E: go 1.21
• chore: bumped to latest go dependencies
• feat: added golangci and pre-commit
• docs: corrections in README and Makefile
• tests: E2E tests timeout properly now
• tests: added failing E2E case when the signature is in another repository and not in the same as the image
• tests: added named returned values for helper method
• sec: min TLS versions and timeout for headers set
• chore: moved same typed variables together in functions
• chore: renamed variables to avoid shadowing packages
• chore: replace len(stringVar) == 0 with stringVar == ""
• perf: replaced some looping over variables with indexing to save some RAM ( 💚 )
• refactor: newKeychainForPod to reduce complexity of Serve function
• chore: replaced int values for codes with the http package's constants