[mlir] DenseStringElementsAttr added to default attribute types

Summary:
Implemented a DenseStringsElements attr for handling arrays / tensors of strings. This includes the
necessary logic for parsing and printing the attribute from MLIR's text format.

To store the attribute we perform a single allocation that includes all wrapped string data tightly packed.
This means no padding characters and no null terminators (as they could be present in the string). This
buffer includes a first chunk of data that represents an array of StringRefs, that contain address pointers
into the string data, with the length of each string wrapped. At this point there is no Sparse representation
however strings are not typically represented sparsely.

Differential Revision: https://reviews.llvm.org/D78600