Support optimistic transactions as an option

**Is your feature request related to a problem? Please describe.**

I found that for some usage patterns optimistic transactions would work better than pessimistic.

**Describe the solution you'd like**

I'd like to pass an option to `runTransaction` which would allow me to choose if pessimistic or optimistic transactions should be used.

**Describe alternatives you've considered**

There are workarounds for it, like manually fetching documents, checking preconditions, then starting a transaction and reading the documents again, but I find it unnecessarily verbose and having an optimistic transaction would make my code simpler.

**Additional context**

None.