Make is_json more permissive

[shend-block]

Currently the documentation states is_json is a infallible function call. However passing in a argument without casting as a string will fail

Running tests
2023-10-10T18:25:59.580454Z ERROR vector::unit_test: Failed to execute tests:
Failed to build test 'test_transforms':
  failed to create test condition '0': 
  error[E630]: fallible argument
    ┌─ :1:9
    │
  1 │ assert!(is_json(.actor))
    │         ^^^^^^^^^^^^^^^
    │         │
    │         this expression can fail
    │         handle the error before passing it in as an argument
    │
    = see documentation about error handling at https://errors.vrl.dev/#handling
    = see language documentation at https://vrl.dev
    = try your code in the VRL REPL, learn more at https://vrl.dev/examples
  .
~/Development/ccd-security-lake shend/okta *+ vector --version
vector 0.33.0 (x86_64-apple-darwin 89605fb 2023-09-27 14:18:24.180809939)

When casting the argument as a string using string! it passes

assert!(is_json(string!(.actor)))

Would it make sense to have is_json() check if the argument as a string and return false if its not?