feat: make segment condition value dynamic

[matthewelwell]

Changes

For self hosted customers, the limit on the length of each segment condition is very restrictive for little benefit. The restriction in our SaaS environment is primarily driven by the size limit that we have imposed on us by dynamodb (although hopefully this will be worked around in the not too distant future).

This PR allows self hosted customers to define the length of that column via an environment variable. I have added documentation warning against it's use for already running instances of Flagsmith.

How did you test this code?

Ran the following:

export DATABASE_URL=<url of an empty database>
export SEGMENT_CONDITION_VALUE_LIMIT=1999999
python manage.py migrate
python manage.py runserver

and validated the following:

1. The database column was created with a max size limit of 1999999B
2. It is possible to create a segment condition with a size exceeding the default limit of 1000B
3. It is not possible to create a segment condition with a size exceeding the configured limit of 1999999B
4. Using a segment with a value that has a size close to the limit still works in a reasonable manner

Note that I also ran this to confirm that django doesn't think there are migrations to run (assuming the environment variable is not modified):

❯ python manage.py makemigrations segments
System check identified some issues:

WARNINGS:
?: (axes.W002) You do not have 'axes.middleware.AxesMiddleware' in your settings.MIDDLEWARE.
No changes detected in app 'segments'

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Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (445dc2b) 95.96% compared to head (06537ef) 76.38%.
Report is 10 commits behind head on main.

❗ Current head 06537ef differs from pull request most recent head d933ec1. Consider uploading reports for the commit d933ec1 to get more accurate results

Files	Patch %	Lines
api/webhooks/webhooks.py	94.23%	3 Missing ⚠️
api/app/settings/common.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3245       +/-   ##
===========================================
- Coverage   95.96%   76.38%   -19.59%     
===========================================
  Files        1062     1033       -29     
  Lines       32297    31254     -1043     
===========================================
- Hits        30993    23872     -7121     
- Misses       1304     7382     +6078     

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[zachaysan]

One small question / suggestion but overall the approach makes sense.