Convert CEL values to JSON

[clarkmcc]

This PR adds support for converting CEL values to JSON values.

1. Enable the JSON feature

   cel-interpreter = { ..., features = ["json"] }

2. Use method on CEL values to convert to JSON

   
   let value = program.execute(&Context::default()).unwrap();
   let json = value.json().unwrap();

[clarkmcc]

CC'ing @jplock @alexsnaps since we've talked around this feature/issue in the past (see #68).

[jplock]

Tested the latest changes in this branch, works great. Thank you for the fix 👍