[mlir] Add a parameter for passing default values to `StringRefParameter`

**For an explanation of these patches see D154153.**

Commit message:
Currently the `StringRefParameter` parameter doesn't support default values,
this patch allows the usage of them, eg:
`StringRefParameter<"description", [{"default_value"}]>`

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D154097