feat: Allow users to customize fetch behavior

[rafaeelaudibert]

NextJS SSR v15.1+ will not cache fetch requests by default, but some users might want that, specially in development mode. We're now allowing them to customize our fetch requests by setting the new fetch_options setting when calling posthog.init

This needs to be used carefully, but it should be extremely powerful and useful for NextJS users. We'll add documentation about it on posthog.com soon.

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

See #1529

[vercel]

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Dec 11, 2024 9:55pm

[github-actions]

Size Change: +753 B (+0.02%)

Total Size: 3.21 MB

Filename	Size	Change
dist/array.full.es5.js	261 kB	+75 B (+0.03%)
dist/array.full.js	364 kB	+75 B (+0.02%)
dist/array.full.no-external.js	363 kB	+75 B (+0.02%)
dist/array.js	178 kB	+76 B (+0.04%)
dist/array.no-external.js	177 kB	+75 B (+0.04%)
dist/main.js	179 kB	+76 B (+0.04%)
dist/module.full.js	364 kB	+75 B (+0.02%)
dist/module.full.no-external.js	363 kB	+75 B (+0.02%)
dist/module.js	178 kB	+76 B (+0.04%)
dist/module.no-external.js	177 kB	+75 B (+0.04%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	206 kB
dist/customizations.full.js	12.6 kB
dist/dead-clicks-autocapture.js	14.4 kB
dist/exception-autocapture.js	9.48 kB
dist/external-scripts-loader.js	2.48 kB
dist/recorder-v2.js	115 kB
dist/recorder.js	115 kB
dist/surveys-preview.js	57.6 kB
dist/surveys.js	63.3 kB
dist/tracing-headers.js	1.75 kB
dist/web-vitals.js	10.3 kB

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