[mlir] Make it possible to build a DenseResourceElementsAttr from untyped memory. (#66009)

Exposes the existing `get(ShapedType, StringRef, AsmResourceBlob)`
builder publicly (was protected) and adds a CAPI
`mlirUnmanagedDenseBlobResourceElementsAttrGet`.

While such a generic construction interface is a big help when it comes
to interop, it is also necessary for creating resources that don't have
a standard C type (i.e. f16, the f8s, etc).

Previously reviewed/approved as part of https://reviews.llvm.org/D157064