[ISTHMUS] Function Signatures Include Outdated opt

[vbarua]

Plans generated with Isthmus currently include opt_ as a prefix in function signatures.

For example

./isthmus -c "CREATE TABLE persons (zip INT)" "SELECT zip + 1 FROM persons"

creates the follow plan (elided for brevity)

{
  "extensionUris": [{
    "extensionUriAnchor": 1,
    "uri": "/functions_arithmetic.yaml"
  }],
  "extensions": [{
    "extensionFunction": {
      "extensionUriReference": 1,
      "functionAnchor": 0,
      "name": "add:opt_i32_i32"
    }
  }],
  ...
}

in which the + operators is named "add:opt_i32_i32"

The current spec for Function Signature Compound Names no longer includes opt.
This was removed as part of substrait-io/substrait#342.

Isthmus should be updated to omit the opt_ prefix to match the spec.

[vbarua]

Related to #18

[vbarua]

Addressed by #124