Add integration tests w/ benchmarks

[oakromulo]

motivation: covers issue https://github.com/lesnitsky/flutter_localstorage/issues/1

expected benefits: additional robustness, peace of mind on new releases and capability to measure performance easily

p.s. the base project structure has been fully respected and left alone, with the exception of the proposed removal of pubspec.lock according to What not to commit

[lesnitsky]

WOW 🤯 Great job @oakromulo, very appreciate it I'll have a closer look before merging as this one seem to be large