profiles: Make mapping in Profile optional

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.

[0]: #534 (comment)

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>

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 is optional.
   repeated Mapping mapping = 3;
   // Locations referenced by samples via location_indices.
   repeated Location location = 4;