shared/crypto: Extend bt_crypto_sign_att with sign counter

shared/crypto: Extend bt_crypto_sign_att with sign counter

Sign counter is use in two places during att signing:
1) Shall be concatenated with message to be sign as specified in BT spec
4.1, Vol[3], Part H, chapter 2.4.5

2) Shall be a part of signature send in the att packet as specified in
BT spec 4.1 Vol[3], Part C, chapter 10.4.1

With this patch, bt_crypto_sign_att returns signature as specified in 2)

This patch also updates unit tests so now it uses sign counter 0. Note
that test vectors no longer match that one from  NIST Special
Publication 800-38B