Fix deserialization of custom stackframe fields in cached error reports

[fractalwrench]

Goal

This changeset fixes the deserialization of custom fields in a stackframe by ErrorReader, when an error report has been cached on disk. This caused several issues:

• Prevented delivery of JS exceptions in React Native where the method was null for some stackframes
• Missing columnNumber in React Native JS exceptions, reducing the available information for source mapping
• Missing loadAddress, frameAddress, and symbolAddress fields for NDK errors
• Stacktrace length was not trimmed for JS/NDK exceptions at the JVM deserialisation layer, meaning this could cause recursive stacktraces to be dropped by our API

Changeset

If no beforeSend callback has been specified, skip deserialization of the error report and stream the file directly, to reduce unnecessary work and mitigate the impact of any other deserialisation bugs.

Generally speaking, the serialization has been changed to allow an arbitrary List<Map<String, Object>>, which contains the full stacktrace. Broken down across classes, this has had the following effects:

CachedThread:

• Instantiates a Stacktrace object eagerly in the constructor, rather than in the stream method
• Adds a secondary constructor which takes an arbitrary list of stackframes.

ErrorReader:

• Deserializes stackframes as a List<Map> rather than mapping data into a StackTraceElement.

Exceptions:

• The toStream method now delegates to the BugsnagException class. The pre-existing implementation has been split across BugsnagException and Stacktrace respectively.

Stacktrace:

• Trim stacktrace length in the constructor rather than in the toStream method
• Add secondary constructor which provides a List<Map> for arbitrary stackframes
• Convert StackTraceElement to List<Map> by adapting the existing serialization code

BugsnagException:

• Implement Streamable. If the wrapped exception is of type Streamable, delegate the streaming to it
• Add a constructor which accepts List<Map> for serializing stackframes
• If a List<Map> has been supplied, construct the Stacktrace using that, rather than the throwable's stacktrace.

Tests

• Added mazerunner scenarios with custom streamable exceptions
• Added and updated existing unit test coverage

[kattrali]

I've gotten to the start of Stacktrace.java so far but need to break up my review a bit. This is what I have so far.

review comments resolved