[macsec] Additional support to configure the pfc_encrypt mode

[judyjoseph]

Support CLI option to configure pfc_encrypt mode by user. This will help in future deployments

pfc_encryption_mode         = "bypass" / "encrypt" / "strict_encrypt"
                                                       ; The behavior to PFC frames.
                                                       ; "bypass": react clear and encrypted PFC frames, send clear PFC frames.
                                                       ; "encrypt": react clear and encrypted PFC frames, send encrypted PFC frames.
                                                       ; "strict_encrypt": only react encrypted PFC frames, send encrypted PFC frames.
                                                       ; If the specified mode cannot be supported by the platform, record an error in the log file.
                                                       ; Default "bypass".