Enable use with CISCO VPN / allow specification of MTU size

[crkurz]

What is the new feature about?

Default MTU=1340 in /wg0.conf in cargo container causes reproducible packet drops when WireGuard connection data is transported over CISCO VPN connection.

gefyra-client should allow overriding MTU size, either on gefyra-cli-create or on gefyra-con-con or both.

Defining on gefyra-cli-create might be best, as this is an admin activity and gefyra admin would know about such MTU requirements (unlike the (many) gefyra users)

Why would such a feature be important to you?

Gefyra is not usable with default MTU on Mac when using CISCO VPN

Anything else we need to know?

Veeery tough to diagnose issue

temp work-around in cargo container:

1. change MTU=1200 in /config/wg0.conf
2. wg-quick down /config/wg0.conf
3. wg-quick up /config/wg0.conf