gre: Add support for GRE keys in the GRE plugin

gre: Add support for GRE keys in the GRE plugin

The added feature enables the GRE plugin to create tunnels between the same endpoints,
distinguishing them by the "key" value. It uses the standard 'key'
parameter in the GRE header.
Changes have been made to add support for CLI and API to create tunnels with a "key" value.

The CLI syntax is as follows:
   create gre tunnel src src_IP_Address dst dst_IP_Address key key_value

All existing GRE functionalities, such as tunnel type and mode, remain unchanged.
GRE key support has been implemented for all non-ERSPAN tunnel types, including both IPv4 and IPv6.
Additionally, modifications were made to the GRE packet header, data structure, and inbound/outbound
packet processing to accommodate key configuration through CLI and API.

Type: feature
Change-Id: I222d585007fa264e7cc12c79d6ba9c63c044f133
Signed-off-by: Masih Nilforoush <[email protected]>