Provide a module for SDK interfaces implemented externally

Is your feature request related to a problem? Please describe.
Currently, any SPI between the SDK and exporters is within the SDK modules.

Having the SPI within the SDK implementation has several impacts:

• There's no API boundary between the SDK and exporters. Exporters can potentially utilize parts of the SDK that weren't intended
• It complicates implementing exporters for those not familiar with the project, as it requires knowing which interfaces/classes from the SDK need to be implemented
• A separate module would make it possible to independently version, resulting in longer compatibility between SDK and exporters if the SPI has not been modified

Describe the solution you'd like
Either a pure SPI module that only contains interfaces or more of a "common" SPI module that includes the interfaces but also common classes that are required by both the SDK and exporters. Whichever approach can be taken will be influenced by the current SDK implementation and how easily some of those "common" SPI pieces can be extracted.

I'm happy to prototype this work if there's interest

Start by mapping the SDK modules and exporter-facing SPI interfaces currently described in the issue. Compare a pure SPI module with a common module, identify which interfaces or shared classes would move, and define the module boundary and compatibility expectations before proposing an implementation.