[lldb] Add an SB API to get progress events as SBStructuredData

This is a preparatory patch to add an SB API to get the progress data as
SBStructuredData. The advantage of using SBStructuredData is that the
dictionary can grow over time with more fields.

This approach is identical to the way this is implemented for diagnostic
events.

Differential revision: https://reviews.llvm.org/D143687