With declarative configuration stable (#4374), I propose adjusting the spec contribution process to be "declarative config first".

What this means: If a spec PR is proposing changes to an SDK component's configuration surface area, it should include a reference to an PR against opentelemetry-configuration to propose corresponding changes to the declarative config schema.

Why this is important:

• Ensures spec PRs are evaluated holistically. The declarative config schema is on track to become a crucial cross-language user facing API for the whole project. Operators can't or don't want to modify source code to use the programmatic API, and env vars are insufficient for non-trivial setups. By doing this, we add UX evaluation criteria in addition to the existing conceptual criteria.
• Ensures declarative config schema stays in sync with the spec.
• Helps drive consistency across language implementations, since its much harder to misinterpret / reinterpret a feature when there's a config schema spec to adhere to.
• Further incentivizes the remaining languages to implement declarative config, helping the project achieve its mission of telemetry concepts being universal across languages.

cc @open-telemetry/configuration-approvers