[ldb/Reproducers] Add YamlRecorder and MultiProvider

This patch does several things that are all closely related:

 - It introduces a new YamlRecorder as a counterpart to the existing
   DataRecorder. As the name suggests the former serializes data as yaml
   while the latter uses raw texts or bytes.

 - It introduces a new MultiProvider base class which can be backed by
   either a DataRecorder or a YamlRecorder.

 - It reimplements the CommandProvider in terms of the new
   MultiProvider.

Finally, it adds unit testing coverage for the MultiProvider, a naive
YamlProvider built on top of the new YamlRecorder and the existing
MutliLoader.

Differential revision: https://reviews.llvm.org/D83441