Validate feasibility of supporting new attribute types

[pellared]

What

Investigate whether it is possible and acceptable to extend the OTel Go API and SDK with new attribute types.

Guidelines for evolving API/SDK: https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4485-extending-attributes-to-support-complex-values.md#how

Prototype in OTel Go demonstrating feasibility of adding new attribute types: #6809

Why

Per open-telemetry/opentelemetry-specification#4602

Follows open-telemetry/opentelemetry-specification#4614

The goal is to make sure that PRs like these are acceptable for OTel Go:

• all: drop attribute values restrictions opentelemetry-proto#707
• [EndGoal] [DoNotMerge] Extend the set of attribute value types opentelemetry-specification#4636

[pellared]

Validated in #6809