Using opentonapi as a package

[dkatzan]

Hi, trying to use this repo as a package in my go code
using go get github.com/tonkeeper/opentonapi results in getting the latest 1.X version which is 1.2.2 from a few months ago

In order to get newest v2 version, the go.mod file must be updated to include v2 in the module name
I have opened this PR for doing so
#450