Introduce `full_cause_backtrace` configuration

[davidtaylorhq]

By default, RSpec only prints the first line of a 'caused by' backtrace. This commit introduces a new 'full_cause_backtrace' config option which will enable printing the entire cause backtrace.

This feature has previously been requested and discussed in #3027. The outstanding questions there were:

• whether the config should be a boolean or an enum. Here I have implemented it as a boolean.
• if/how to expose this on the CLI. I have omitted this entirely, but am happy to work on this if it's a pre-requisite for merging

[pirj]

Thank you, @davidtaylorhq !

[JonRowe]

Released in 3.13.0.