Support generic sets

[qdeslandes]

Sets keys are defined statically: set.h contains an enumeration of valid keys, and each set type has its logic supported in the source code.

This behaviour makes sets very rigid and even useless for some users: either your key type is supported, or you need to rely on slower rules processed sequentially.

Users should be able to define set keys themselves using one or more matcher keys, for example:

chain BF_HOOK_XDP{ifindex=2} policy ACCEPT
    set
        name test_set
        key ip4.saddr ip4.dport
        values { 192.168.1.1, 22 }, { 192.168.1.1, 53 }
  rule
      [...]