Create a `NewTransaction` function

Currently all the functions of onyx take in a `*badger.Txn` as their final argument which lets the user pass in a transaction under which the graph operation should be executed. However, currently there is no way for users of Onyx to actually create a new `*badger.Txn` without calling `graph.DB.NewTransaction()`, which requires the `DB` field of the `Graph` struct to be publicly exposed, which is not ideal.

Create a function `NewTransaction` on the `Graph` struct which returns a `*badger.Txn` for the users to use. Note that `NewTransaction` should take in a boolean which tells it if a read-only txn or a read-write txn.

Remember to create a unit test for this function!

_psst, remember to update the usage guide in the README too_ 👀