Manage sketch attributes in the CLI client

[jaegeral]

This PR:

• adds several capabilities in the CLI client to interact (list, add, remove) with sketch attributes timesketch sketch attributes.
• It also slightly increases the data returned for timesketch sketch describe
• adds some information for developers who the target audience of the CLI client is
• adds errors in the sketch command when asked for output formats that are not implemented (yet)

These attributes are e.g. used to store intelligence, but can also be used to store ticket ids etc.

The new list attributes has list, add and remove commands allow you to manage attributes in Timesketch.

• The attributes command lists all of the attributes related commands in Timesketch.

• The list command lists all attributes in a sketch

• The add command adds a new attribute to Timesketch.

• The remove command removes an existing attribute from Timesketch.

• [x] Make it clear if an output format is not implemented

For more information on these commands, please see the timesketch-cli documentation changed with this PR.

Some examples:

timesketch --sketch 2 sketch attributes list
{'intelligence': {'ontology': 'intelligence', 'value': {'data': [{'externalURI': 'google.com', 'ioc': '1.2.3.4', 'tags': ['foo'], 'type': 'ipv4'}, {'externalURI': 'fobar.com', 'ioc': '3.3.3.3', 'tags': ['aaaa'], 'type': 'ipv4'}]}}, 'ticket_id': {'ontology': '12345', 'value': 'text'}}

timesketch --sketch 2 sketch attributes add --name ticket_id2 --ontology text --value 12345
Attribute added: ticket_id2 text 12345

Another example:

timesketch --sketch 2 --output-format json sketch attributes list | jq '.'
{
  "intelligence": {
    "ontology": "intelligence",
    "value": {
      "data": [
        {
          "externalURI": "google.com",
          "ioc": "1.2.3.4",
          "tags": [
            "foo"
          ],
          "type": "ipv4"
        },
        {
          "externalURI": "fobar.com",
          "ioc": "3.3.3.3",
          "tags": [
            "aaaa"
          ],
          "type": "ipv4"
        }
      ]
    }
  },
  "ticket_id": {
    "ontology": "12345",
    "value": "text"
  },
  "ticket_id2": {
    "ontology": "12345",
    "value": "text"
  },
  "ticket_id3": {
    "ontology": "12345",
    "value": "text"
  }
}