Add support for VXLAN and VLAN (#1051)

Added support for capturing virtualized traffic.

## VXLAN
https://en.wikipedia.org/wiki/Virtual_Extensible_LAN

VXLAN implemented as separate engine, which opens UDP socket and awaits traffic. 
This approach is made to work with AWS Traffic Mirroring.

In order to enable VXLAN set `--input-raw-engine vxlan`

Example:
```
gor --input-raw :80 --input-raw-engine vxlan --output-stdout`
```

By default, it looks for vxlan traffic on the standard 4789 port, but you can override it with `--input-raw-vxlan-port`. 
Additionally, you can allow only specific VNIs using `--input-raw-vxlan-vni`, or disallow by using the same option, but by adding "minus" sign to the value: `--input-raw-vxlan-vni -2`. 

Example with all options:
```
gor --input-raw :80 --input-raw-engine vxlan --input-raw-vxlan-vni 1 --input-raw-vxlan-vni 2 --input-raw-vxlan-port 2222 --output-stdout
```

# VLAN
https://en.wikipedia.org/wiki/IEEE_802.1Q

VLAN protocol enabled using `--input-raw-vlan` argument, and you can filter for specific VLAN VIDs using `--input-raw-vlan-vid`. VLAN filtering happens on BPF level.

Example:
```
gor --input-raw :80 --input-raw-vlan --input-raw-vlan-vid 1 --output-stdout`
```

## Notes

Did a refactoring of RAW Input options, so it will be easy to extend in future.

docs/Capturing-and-replaying-traffic.md

@@ -37,6 +37,11 @@ sudo gor --input-raw :80 --input-raw-engine "raw_socket" --output-http "http://s
 
 You can read more about [[Replaying HTTP traffic]].
 
+You can use VXLAN or traffic mirroring from AWS to capture the traffic. The 4789 UDP port will be opened and that works as you are launched GoReplay on the source machine.
+
+```
+gor --input-raw :80 --input-raw-engine vxlan -output-stdout
+```
 
 ### Tracking original IP addresses
 You can use `--input-raw-realip-header` option to specify header name: If not blank, injects header with given name and real IP value to the request payload. Usually, this header should be named: `X-Real-IP`, but you can specify any name.