Add clustering to scattermapbox

[elben10]

Supersedes #5147.

I've added a trace that allow one to use mapbox clustering layer.

Currently it doesn't support the fill, selection and text features.

TODO:

• add jasmine test for hover
• add jasmine tests to ensure Plotly.react can switch between modes i.e. with/without cluster
• add second mock & baseline using new attributes e.g. step and opacity

[archmoj]

Thanks @elben10 for opening this PR.
I updated the description and added various test to the TODO list mainly to ensure the correct functionality would be maintained in future.

[archmoj]

@elben10 Before adding tests please sync the master branch of your fork with plotly/plotly.js.
Then run git merge upstream/master into your branch.
Thank you!

[nicolaskruchten]

A small update on timing: our team is working hard on releasing v2.0 of Plotly.js, which we anticipate will happen in early April. This PR would be a good candidate to land in the library in v2.1 or later, so with apologies for the delay, we will likely not be able to give much feedback on this PR for the next few weeks :)

[archmoj]

There is a conflict in test/jasmine/tests/mock_test.js
@elben10 Wondering if you could possibly pull upstream/master and resolve the conflict by accepting incoming changes please?

[archmoj]

@elben10 Are you planning to reopen this interesting PR after resolving that little conflict?
If needed I'd be happy to help : )

[elben10]

Sure

[archmoj]

💃
Merging into finalist-cluster-scattermapbox branch.
Thanks @elben10 for the hard work 🏆 🥇
I'll reopen a PR to target master after resolving the conflicts and adjusting the tests.

[elben10]

Sounds great. Thanks for the guidance and support! 🥇 👍