Stabilize key declarative config APIs

[jack-berg]

Especially DeclarativeConfiguration.parse and DeclarativeConfiguration.create, which are the key operations referenced in the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk.md#sdk-operations

Since parse returns OpenTelemetryConfigurationModel, the in-memory representation of the configuration model, that means that all the generated types from the schema also need to be made public.

[ADITYA-CODE-SOURCE]

@jack-berg I'd be interested in helping with this. Could you clarify the scope is it mainly about changing access modifiers on specific classes and methods, or does it involve deeper design decisions? If the scope is well-defined, I'd like to take it.