Bluetooth: tester: Add command pair_v2 for BTP GAP

Compare with BTP GAP pair, the pair_v2 add more arguments, including
security mode, security level, and flags.

The argument `security mode` is used to set the security mode.

The argument `security level` is used to set the security level of the
specific security mode.

The argument `flags` is used to add additional setting, such as flag
`BTP_GAP_PAIR_V2_FLAG_FORCE_PAIR` is used to force the pairing
procedure.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>