Added poll event deserialization

Got a chance to experience the suffering of implementing deserialization of matrix events 😛

The MSC implementation (https://github.com/matrix-org/matrix-spec-proposals/pull/3381) is somewhat messy because there are two different event formats for stable and unstable version of events, and opted to implement handling of both for the following reasons:

1. To follow similar implementation structure and support level in Rust/Android SDKs for reference
2. Based on comments in the MSC PR, seems like this may be merged into the spec soon.
3. For backward compatibility, we'll have to be supporting unstable events even after SDKs migrate to using the stable versions .

If you prefer to go cave-man to simplify things by supporting only 1 version or not fully complying with the spec, let me know.