Move vendored code into a `vendored` package

### Description

Currently the SDK has vendored code in lots of places.
We could move that to a vendor package, e.g. `io.sentry.vendor`.

We already do this for `io.sentry.vendor.gson`.

There are a lot more vendored classes we could move.

This is a breaking change since the classes are public and could be used by customers. Therefore we should change this in a major.