Add a converter for classpaths.proto between proto and json/text formats.

The proto is used at build time to define classpath fragments to be read
and parsed at runtime by `derive_classpath` service; in order to define
and set BOOTCLASSPATH, DEX2OATCLASSPATH, and SYSTEMSERVERCLASSPATH
environ variables.

The tool is used for debugging and generation of binary protos.

Bug: 180105615
Test: m && launch_cvd with a follow up changes
Change-Id: I7e8b084e5f63e9aaad11da3221b909818e69e235