Make some methods open to allow custom Throwable format

The default handling of `Throwable` is to print the full stack, which may be too large.

Actually, I don't think `prepareLog` should append the throwable to the message so that the subclass can decide what to do with them.

But in order not to break the behavior, could you make `prepareLog()` or `getStackTraceString()` open and protecte? Then I can change the format.

Start by locating prepareLog() and getStackTraceString() in the Timber source and inspect how Throwable values are currently included in log messages. Make the relevant methods extensible without changing the default full-stack behavior, then verify the existing logging behavior and custom formatting path.

Written by the indexing model from the issue text.