Add support for title argument in CreateChannelCanvas method

[jarospisak-unity]

This PR adds support for title argument in the CreateChannelCanvas method.

Closes #1482.

[nlopes]

Hi @jarospisak-unity. As it stands this is a breaking change and I'd rather not add breaking changes if they can be avoided at all.

One way is simply to add a method that allows to pass options. This pattern already exists in other parts of the codebase. Happy for you to give it a go if you're ok to do so, alternatively, I'll come back to this soon-ish.

func (api *Client) CreateChannelCanvasWithOptions(channel string, options
  ...CreateChannelCanvasOption) (string, error) {
      return api.CreateChannelCanvasWithOptionsContext(context.Background(), channel,
  options...)
  }
[...]

[jarospisak-unity]

@nlopes thanks for the feedback, I wasn't aware of that pattern. I'll refactor this pull request.