profiles: align type of index into string table (#557)

With `Mapping.filename`, `Function.name`, `Label.key` and others the type of the index into the string table is always of type `int64`.
For consistency align the type of `Location.type_index`, which is also an index into the string table, to `int64`.

Co-authored-by: Tigran Najaryan <4194920+tigrannajaryan@users.noreply.github.com>

opentelemetry/proto/profiles/v1experimental/pprofextended.proto

@@ -355,7 +355,7 @@ message Location {
   bool is_folded = 5;
 
   // Type of frame (e.g. kernel, native, python, hotspot, php). Index into string table.
-  uint32 type_index = 6;
+  int64 type_index = 6;
 
   // References to attributes in Profile.attribute_table. [optional]
   repeated uint64 attributes = 7;