[C++][Gandiva] Allow searching function signatures by name

We need to get all signatures of a function name during the type inference stage of Gandiva parser.

**Reporter**: [Jin Shang](https://issues.apache.org/jira/browse/ARROW-17746) / @js8544

_{**Note**: *This issue was originally created as [ARROW-17746](https://issues.apache.org/jira/browse/ARROW-17746). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by tracing the Gandiva parser's type-inference stage and how function signatures are currently looked up. Identify the signature registry or lookup entry point, then verify that a function name can return all matching signatures for inference.

Written by the indexing model from the issue text.