Add a command to test plugin compatibility with the krakend binary

[kpacha]

./krakend test-plugin -h
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Version: 2.6.0

Tests that one or more plugins are loadable into KrakenD.

Usage:
  krakend test-plugin [flags] [artifacts]

Examples:
krakend test-plugin -scm ./plugins/my_plugin.so ./plugins/my_other_plugin.so

Flags:
  -c, --client     The artifact should contain a Client Plugin.
  -h, --help       help for test-plugin
  -m, --modifier   The artifact should contain a Req/Resp Modifier Plugin.
  -s, --server     The artifact should contain a Server Plugin.

Ex:

./krakend test-plugin -smc ../../luraproject/lura/transport/http/client/plugin/tests/lura-client-example.so
[KO] SERVER	lura-client-example.so: The plugin does not contain a HandlerRegisterer.
[KO] MODIFIER	lura-client-example.so: The plugin does not contain a ModifierRegisterer.
[OK] CLIENT	lura-client-example.so
[KO] 1 tested plugin(s) in 12.498341ms.
1 plugin(s) failed.