Polaris SPI definitions

The current Evolution page[1] lacks a formal definition for Service Provider Interfaces (SPIs) regarding Java interfaces.

As Polaris has matured and now used by multiple adopters, establishing a formal SPI is critical. Without clearly defined public interfaces, downstream users and third-party integrations are at risk of breaking changes.

We need to explicitly define Polaris SPIs to ensure that all public-facing interfaces strictly adhere to a standardized deprecation and evolution process. This will provide stability for the ecosystem and clear expectations for contributors.

Reference:
1. Current Evolution page: https://polaris.apache.org/in-dev/unreleased/evolution/
2. Sprint discussion: https://docs.google.com/document/d/1cUg4HnUGDN0JKMr0JWQTaKkgzSXENHZ4VMuyRfJ9oTQ/edit?tab=t.lzwxdgyu5e82#heading=h.5zojeahif6e