feat: add UserSignUpMeta model

[matthewelwell]

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Changes

As part of a request from the marketing team we need to capture, and send to Hubspot, information about how the user arrived at Flagsmith when a new user signs up. For example, when clicking on a google ad, the website receives a query string that the FE then adds to a cookie.

This PR adds logic to retrieve the data from that cookie and store it in a new model which is then (TODO) used to send the correct attribution data to Hubspot.

How did you test this code?

TODO

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[github-actions]

Uffizzi Ephemeral Environment deployment-52680

☁️ https://app.uffizzi.com/github.com/Flagsmith/flagsmith/pull/4113

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[zachaysan]

Overall approach looks great. I'd have marked this PR as approved but according to the PR description there are testing and other TODOs that need to land first.

[zachaysan]

It's too bad we can't use a json field here due to other databases

[codecov]

Codecov Report

Attention: Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.44%. Comparing base (1852d94) to head (c784a9d).
Report is 11 commits behind head on main.

Files	Patch %	Lines
api/custom_auth/serializers.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4113      +/-   ##
==========================================
- Coverage   96.44%   96.44%   -0.01%     
==========================================
  Files        1149     1150       +1     
  Lines       37596    37669      +73     
==========================================
+ Hits        36259    36329      +70     
- Misses       1337     1340       +3     

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[matthewelwell]

Closing as this was a failed attempt at getting around Hubspots uselessness. We will need to develop a custom form in hubspot and use that to create the contacts instead of using their native APIs.