feat: introduce varchar<L> type

[dshaaban01]

This PR introduces the varchar compound type, which represents a variable-length Unicode string of up to L characters.

[ingomueller-net]

Take the length from the type, not from the value.

[dshaaban01]

@ingomueller-net I made some thoughts about the creation of a VarCharAttr with the goal of implementing a mechanism that verifies that the length of the string is at most L characters.

This is what I came up with:

• If we have a string "hello" (5 characters), but declare it as type VarChar<6> (can be at most 6 characters, then this means that the same string "hello" declared as type VarChar<5> is different. So I thought it would make sense to add the VarChar type as parameter to the VarChar attribute type. I didn't know any other way of verifying that the length of the string e.g. "hello" is at most L e.g. 6 characters.
• Furthermore, I guess its not statically typed anymore so I have it as a TypeInterferableAttrInterface.

Let me know if this is overkill or if I should add this (or elements of it).

def Substrait_VarCharAttr
    : Substrait_Attr<"VarChar", "var_char", [TypeInferableAttrInterface]> {
  let summary = "Substrait varchar type";
  let description = [{
    This type represents a substrait varchar attribute type, namely a unicode
    string of at most L characters.
  }];
  let parameters = (ins "StringAttr":$value, "VarCharType":$type);
  let assemblyFormat = [{ `<` $value `,` $type `>` }];
  let genVerifyDecl = 1;
}

LogicalResult mlir::substrait::VarCharAttr::verify(
  llvm::function_ref<mlir::InFlightDiagnostic()> emitError, StringAttr value, VarCharType type) {
  if (static_cast<int32_t>(value.getValue().size()) > type.getLength())
    return emitError() << "value length exceeds the maximum length of " << type.getLength();
  return success();
}