feat: GTP-U tunnel support

[mtirpak]

Issue #1546

This pull request adds GTP-U tunnel support to the interface plugin, the related part of the proto file change is as follows. The implementation is based on the VxLan and GRE tunnels.

message GtpuLink {
    enum NextNode {
        // The default next node is l2-input
        DEFAULT = 0;
        // l2-input
        L2 = 1;
        // ip4-input
        IP4 = 2;
        // ip6-input
        IP6 = 3;
    };
    // Source VTEP address
    string src_addr = 1;
    // Destination VTEP address
    string dst_addr = 2;
    // Name of multicast interface
    string multicast = 3;
    // Tunnel endpoint identifier
    uint32 teid = 4;
    // VRF id for the encapsulated packets
    uint32 encap_vrf_id = 5;
    // Next VPP node after decapsulation
    NextNode decap_next = 6;
}

There is a VPP bug in the dump gtpu interface api, which breaks the functionality currently. With VPP 20.01, the minimal version to use this feature is ligato/vpp-base:20.01-rc0.496-g00b2d74d1. (https://gerrit.fd.io/r/c/vpp/+/22904)
The latest VPP 19.08 docker image already includes the fix. (https://gerrit.fd.io/r/c/vpp/+/23054)

[codecov]

Codecov Report

Merging #1560 into dev will increase coverage by 2.68%.
The diff coverage is 73.41%.

@@            Coverage Diff             @@
##              dev    #1560      +/-   ##
==========================================
+ Coverage   53.06%   55.74%   +2.68%     
==========================================
  Files         640      352     -288     
  Lines       77563    30280   -47283     
==========================================
- Hits        41156    16880   -24276     
+ Misses      33913    12075   -21838     
+ Partials     2494     1325    -1169

Flag	Coverage Δ
#e2e	?
#unittests	55.74% <73.41%> (+0.27%)	⬆️