btif_storage: Add way to store GATT server supported features

This is needed for storing Enhanced ATT support.

Tag: #feature
Bug: 159786353
Sponsor: jpawlowski@
Test: compile & manual testing
Bug: 191313013

Merged-In: Ic37b668b91ab6c830d780f70db703a5d9be11677
Change-Id: Ic37b668b91ab6c830d780f70db703a5d9be11677