Add possibility to disable/enable SNMP at runtime

User Story

As a host application, I want to dynamically enable/disable the SNMP protocol running on the netX, so that I can adapt to the end user's security requirements.

Action Items

• A new vendor specific attribute 770 shall be added to the TCP/IP Interface object (0xF5) that enables the host application to dynamically enable/disable the SNMP protocol (default: enabled).
• The new attribute shall be part of the remanent data.
• The new attribute shall be of type BOOL
  • true: SNMP enabled
  • false: SNMP disabled
• When modifying the new attribute (set attribute single) also Bit 1 of the LLDP Datastore attribute 4 (LLDP Management  Object) shall be modified.
• The new attribute shall be settable/gettable from the host and from the network. 
• Modification of the new attribute shall take affect immediately.
• The Protocol API Manual shall describe the new attribute and its behavior. 
• The new attribute shall be mapped to the lwIP functions:
  • LwIP_Snmp_EnableService()
  • LwIP_Snmp_DisableService()
• Add footnote to data sheet that states the possibility to disable SNMP (footnote at "Supported features": SNMP).