OTEP: Extending attributes to support complex values

[lmolkova]

Related to #4468, #4414

This OTEP suggests a path forward on how to support complex attributes. It proposes to:

• allow complex attributes on all signals by default on the API and SDK level
• discourage them on metrics, resources, instrumentation scope, and identifying attributes on entities. If complex attribute is provided, it's probably by mistake, but we let backend decide.
• provide guidance in semconv to default to flat attributes whenever possible and develop conventions with an assumption that complex attributes are not indexed, not queryable and not efficient.
• documents how limits can apply to complex attributes (leaf nodes are counted towards the attribute count limit and are truncated based on the value limit) and suggests other ways to protect from users misusing complex attributes.

Having experimental phase for complex attributes limits the impact of this change and gives backends a graceful period to update.

[pellared]

In general, LGTM. There are still some things that would be good to have clarified.