shared/att: Add ext_signed flag to be able to use external crypto

Due to the issues with CSRK tests ext_signed flag has been added to
allow android/gatt handle signed write operations.

This patch fixes following tests:
GAP__TC_SEC_CSIGN_BV_02_C
GAP__TC_SEC_CSIGN_BI_01_C
GAP__TC_SEC_CSIGN_BI_02_C
GAP__TC_SEC_CSIGN_BI_03_C
GAP__TC_SEC_CSIGN_BI_04_C
SM__TC_SIGN_BV_03_C
14 files changed