Support maps and heterogeneous arrays as attribute values

Resolves #376

Use cases where this is necessary or useful:

1. Specify more than one resource in the telemetry: #579
2. Data coming from external source, e.g. AWS Metadata: #596 (comment)
   or #376 (comment)
3. Capturing HTTP headers: #376 (comment)
4. Structured stack traces: #2841
5. Payloads as attributes: open-telemetry/oteps#219 (comment)

This is a draft PR to see what the change looks like.

If this PR is merged it will be nice to follow it up with:
- A standard way of flattening maps and nested objects when converting from OTLP
  to formats that don't support maps/nested objects.
- Recommendations for semantic conventions to use/not use complex objects.

CHANGELOG.md

@@ -35,6 +35,9 @@ release.
 
 ### Common
 
+- Support maps and heterogeneous arrays as attribute values
+  ([#](https://github.com/open-telemetry/opentelemetry-specification/pull/))
+
 ## v1.14.0 (2022-10-04)
 
 ### Context