profiles: Make mapping in Profile optional (open-telemetry#556)

As commented in [0] and discussed in the OTel Profiling SIG meeting, there are situations where a main binary for a Profile can not be identified. For these cases mark the field optional.

FYI: @brancz @petethepig @open-telemetry/profiling-maintainers 

[0]: open-telemetry#534 (comment)

opentelemetry/proto/profiles/v1development/pprofextended.proto

@@ -79,6 +79,8 @@ message Profile {
   repeated Sample sample = 2;
   // Mapping from address ranges to the image/binary/library mapped
   // into that address range.  mapping[0] will be the main binary.
+  // If multiple binaries contribute to the Profile and no main
+  // binary can be identified, mapping[0] has no special meaning.
   repeated Mapping mapping = 3;
   // Locations referenced by samples via location_indices.
   repeated Location location = 4;