Stabilize ConfigProvider

Is your feature request related to a problem? Please describe.

Stabilize ConfigProvider once declarative config itself is stable.

Describe the solution you'd like

getConfigProvider() is moved from ExtendedOpenTelemetry to OpenTelemetry.

Additional context

Right now, getConfigProvider() is non-null, because it's part ExtendedOpenTelemetry, which is only used in declarative config.

We have to decide what to do for non-declarative config

1. return null
2. return an empty object

• better usability over null
• based on initial discussion, it seems to make sense to have a config provider when not using declarative config, e.g. the agent currently does that

4. return an object to access config properties where applicable (in auto-config)

• could be used in contrib extensions that currently use the declarative config bridge
• not clear if this should better live in SDK or contrib

I created a PoC to show how it could work: https://github.com/open-telemetry/opentelemetry-java/pull/7960

Start by reviewing getConfigProvider() on ExtendedOpenTelemetry and the proposed move to OpenTelemetry, then read the PoC in pull request 7960. Compare the behavior needed for declarative and non-declarative configuration and clarify the intended API contract. Done means the chosen behavior is agreed and the provider API is stabilized consistently.