[APFloat] Add APFloat support for FP6 data types (#94735)

This patch adds APFloat type support for two FP6 data types,
E2M3 and E3M2. The definitions for the two formats are detailed
in section 5.3.2 of the OCP specification, which can be accessed here:

https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf



Signed-off-by: Durgadoss R <durgadossr@nvidia.com>