[Feature Request] C-API: Implement KernelInfoGetAttribute for all possible attribute types

Describe the feature request

The current C-API only offers the following functions for loading attributes:

• KernelInfoGetAttribute_float
• KernelInfoGetAttribute_int64
• KernelInfoGetAttribute_string
• KernelInfoGetAttributeArray_float
• KernelInfoGetAttributeArray_int64
• KernelInfoGetAttribute_tensor

The standard allows the following types:

  enum AttributeType {
    UNDEFINED = 0;
    FLOAT = 1;
    INT = 2;
    STRING = 3;
    TENSOR = 4;
    GRAPH = 5;
    SPARSE_TENSOR = 11;
    TYPE_PROTO = 13;

    FLOATS = 6;
    INTS = 7;
    STRINGS = 8;
    TENSORS = 9;
    GRAPHS = 10;
    SPARSE_TENSORS = 12;
    TYPE_PROTOS = 14;
  }

The following functions are therefore missing from the C-API and should be added:

• KernelInfoGetAttribute_graph
• KernelInfoGetAttribute_sparse_tensor
• KernelInfoGetAttribute_type_proto
• KernelInfoGetAttributeArray_string
• KernelInfoGetAttributeArray_tensor
• KernelInfoGetAttributeArray_graph
• KernelInfoGetAttributeArray_sparse_tensors
• KernelInfoGetAttributeArray_type_proto
• KernelInfoGetAttribute_undefined (?)

Describe scenario use case

Several of the above attribute types are used in the standard. Others might be useful to C-API authors precisely because the operator that would use them is not in the standard. Either way, it is an unfortunate surprise for C-API users to find out that the API does not support an attribute type they were planning to use.

Start by reviewing the existing KernelInfoGetAttribute_* C-API functions and the linked ONNX AttributeType definition. Map the listed scalar and array attribute types to corresponding API functions, resolve whether UNDEFINED needs support, and verify that all standard types are covered.