gadgets/run: Print string representation of enums

[eiffel-fl]

Hi.

This PR prints the string representation of enums.
Note that, it works only on named enum.

You can test it with the following patch:

diff --git a/gadgets/snapshot_socket/program.bpf.c b/gadgets/snapshot_socket/program.bpf.c
index cf790793..99ab7af9 100644
--- a/gadgets/snapshot_socket/program.bpf.c
+++ b/gadgets/snapshot_socket/program.bpf.c
@@ -49,7 +49,7 @@
 struct socket_entry {
        struct gadget_l4endpoint_t src;
        struct gadget_l4endpoint_t dst;
-       __u32 state;
+       enum tcp_conntrack state;
        __u32 ino;
 };

$ sudo -E ./ig run sockets -o columns=state,state_raw        francis/enum-string *% u=
INFO[0000] Experimental features enabled                
STATE                                                                STATE_RAW                                                           
TCP_CONNTRACK_TIME_WAIT                                              7                                                                   
TCP_CONNTRACK_TIME_WAIT                                              7                                                                   
TCP_CONNTRACK_MAX                                                    10                                                                  
TCP_CONNTRACK_MAX                                                    10                                                                  
TCP_CONNTRACK_MAX                                                    10                                                                  
TCP_CONNTRACK_MAX                                                    10

Best regards.

[mauriciovasquezbernal]

LGTM. Thanks for working on this.

[eiffel-fl]

Thank you for the initial code and the review!