gatt: Allow services to indicate eatt support

With this patch eatch client/server can decide if EATT support
should be enabled or not.
If eatt_support is set to true, and EATT is available, it will be used for services added.
If eatt_support is set to false, that means only ATT (CID 4) is used for services added.

Tag: #feature
Bug: 159786353
Sponsor: jpawlowski@
Test: compilation
Change-Id: I584172800ce32a4a11df465841d2bc49ab224939