[Figure-Factory Dendrogram] Adding support for truncate_mode.

[lpinilla]

Now the user can specify via an argument if they want this mode and they can also specify hovertext, color_threshold, show_leaf_counts, show_contracted, leaf_rotation, leaf_font_size. If the truncate mode is on, then showticklabels will be set to False, else, it will remain True.

The end result looks like this:

Code PR

• [x] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• [ ] I have added tests (if submitting a new feature or correcting a bug) or modified existing tests.
• [ ] For a new feature, I have added documentation examples in an existing or new tutorial notebook (please see the doc checklist as well).
• [ ] I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• [ ] For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

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