feat: add optional refs support for synth inputs

[snguyen64]

Continuation of optional refs pr part 2:

• Add Optional field to Ref type in Synthesizer CRD
• Executor skips missing optional inputs without error
• Pass optional metadata via FunctionConfig.optionalRefs
• Function framework checks FunctionConfig for optional status
• Watch controller handles optional ref deletion correctly
• Add comprehensive tests for optional refs functionality

We have previously added this field in the CRD and controller/watcher. These parts function as expected, but eno has the Executor and Synthesizer/Function which was missing the optional refs.

Testing Done:

• unit tests
• standalone testing

Manual Testing Steps

• Build eno controller/reconciler
• Deploy onto a standalone environment
• Deploy synthesizer with optional input that does not exist - confirm no errors
• Update so input exists - confirm synthesizer reads optional input
• Delete optional input - confirm synthesizer still synthesizes without the input