feat: rename KServe Inference configuration to use RuntimeImage instead of Runtime

[RK181]

This pull request updates the handling of KServe inference runtimes by renaming the runtime field to runtime_image throughout the codebase and documentation. This change clarifies the field's purpose and usage, ensuring consistency and better alignment with KServe's terminology. The documentation and validation logic are also updated to reflect this change.

API and Field Renaming:

• Renamed the runtime field to runtime_image in the KserveInference struct, updating all references, comments, and JSON tags accordingly to clarify that this field is for specifying a custom KServe runtime image.
• Updated validation and equality checks to use runtime_image instead of runtime, ensuring correct behavior when comparing or updating KServe inference configurations. [1] [2]

Documentation Updates:

• Updated the docs/fdl.md documentation to replace references to the runtime field with runtime_image, providing clearer guidance on when and why to use this field. The documentation now also explains the relationship between model formats and runtimes.

Spec Generation Logic:

• Modified KServe inference service spec generation and update logic to use the new runtime_image field when setting the runtime in the model spec. [1] [2]

Test Updates:

• Updated tests to use runtime_image instead of runtime when verifying that runtime changes are correctly detected as errors.