String Equality [Feature Request]

### Describe the feature request

With the introduction of string equality in opset 19 (https://github.com/onnx/onnx/blob/main/docs/Operators.md#Equal) and given how useful string equality is for text based models and data science preprocessing, it would be good to add this to onnxruntime.

### Describe scenario use case

Preprocessing string tensors in data science tasks often requires string equality for filtering, data cleaning, etc. In text based models there are numerous use cases as well, especially where outputs aren't categorical options but rather dynamic strings.

Start by reviewing the linked ONNX Equal operator documentation and the ONNX Runtime areas responsible for operator support. Define the required string-tensor equality behavior for filtering, data cleaning, and text-model scenarios, then verify that the runtime supports the requested feature.

Written by the indexing model from the issue text.