api: vat2 and json autogeneration for api messages

api: vat2 and json autogeneration for api messages

VAT2: A completely auto-generated replacement of VAT.
Reads input message in JSON from stdin and outputs received messages in JSON.

A VAT2 plugin is automatically built for a .api file.
There no longer a need for a separate _test.c.

Example:
vat2 show_version {}
{
        "_msgname":     "show_version_reply",
        "retval":       0,
        "program":      "vpe",
        "version":      "21.01-rc0~411-gf6eb348a6",
        "build_date":   "2020-11-19T09:49:25",
        "build_directory":      "/vpp/autogen3"
}

vat2 sw_interface_dump '{"sw_if_index": -1,
                         "name_filter_valid": 0,
                         "name_filter": ""}'
[{
                "_msgname":     "sw_interface_details",
                "sw_if_index":  0,
                "sup_sw_if_index":      0,
                "l2_address":   "00:00:00:00:00:00",
                "flags":        "Invalid ENUM",
                "type": "IF_API_TYPE_HARDWARE",
                "link_duplex":  "LINK_DUPLEX_API_UNKNOWN",
                "link_speed":   0,
                "link_mtu":     0,
                "mtu":  [0, 0, 0, 0],
                "sub_id":       0,
                "sub_number_of_tags":   0,
                "sub_outer_vlan_id":    0,
                "sub_inner_vlan_id":    0,
                "sub_if_flags": "Invalid ENUM",
                "vtr_op":       0,
                "vtr_push_dot1q":       0,
                "vtr_tag1":     0,
                "vtr_tag2":     0,
                "outer_tag":    0,
                "b_dmac":       "00:00:00:00:00:00",
                "b_smac":       "00:00:00:00:00:00",
                "b_vlanid":     0,
                "i_sid":        0,
                "interface_name":       "local0",
                "interface_dev_type":   "local",
                "tag":  ""
        }]

This is the first phase and vat2 is not integrated in packaging yet.

Type: feature
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: Ib45ddeafb180ea7da8c5dc274a9274d7a4edc876
Signed-off-by: Ole Troan <ot@cisco.com>