[mlir][OpDSL] Add type function attributes.

Previously, OpDSL operation used hardcoded type conversion operations (cast or cast_unsigned). Supporting signed and unsigned casts thus meant implementing two different operations. Type function attributes allow us to define a single operation that has a cast type function attribute which at operation instantiation time may be set to cast or cast_unsigned. We may for example, defina a matmul operation with a cast argument:

```
@linalg_structured_op
def matmul(A=TensorDef(T1, S.M, S.K), B=TensorDef(T2, S.K, S.N), C=TensorDef(U, S.M, S.N, output=True),
    cast=TypeFnAttrDef(default=TypeFn.cast)):
  C[D.m, D.n] += cast(U, A[D.m, D.k]) * cast(U, B[D.k, D.n])
```

When instantiating the operation the attribute may be set to the desired cast function:

```
linalg.matmul(lhs, rhs, outs=[out], cast=TypeFn.cast_unsigned)
```

The revsion introduces a enum in the Linalg dialect that maps one-by-one to the type functions defined by OpDSL.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D119718