Add hash parameter to GET/agents/groups/:group_id/files API call

[mgmacias95]

Hi team,

This PR adds a hash parameter to GET/agents/groups/:group_id/files API call. This parameter allows selecting the hash algorithm which will be used to calculate every group file checksums.

Some examples:

# curl -u foo:bar "localhost:55000/agents/groups/default/files?pretty&hash=sha256&limit=1"
{
   "error": 0,
   "data": {
      "totalItems": 24,
      "items": [
         {
            "hash": "d76908d51018ec72afc1a7e17fbc3971c6a812446fd930fdba5ed66f1af47ed0",
            "filename": "agent.conf"
         }
      ]
   }
}
# curl -u foo:bar "localhost:55000/agents/groups/default/files?pretty&hash=aaaa"
{
   "error": 1723,
   "message": "Hash algorithm not available: Available algorithms are SHA1, SHA224, SHA384, ecdsa-with-SHA1, SHA256, SHA512, md5, sha1, sha224, dsaEncryption, DSA, MD5, sha384, sha256, sha512."
}

Best regards,
Marta