Add support to opaque elements attributes

For some of the constant vector / tesor, if the compiler doesn't need to
interpret their elements content, they can be stored in this class to save the
serialize / deserialize cost.

syntax:

`opaque<` tensor-type `,` opaque-string `>`

opaque-string ::= `0x` [0-9a-fA-F]*
PiperOrigin-RevId: 218399426