chore(deps): bump flagsmith sdk to 3.0

[Totodore]

This PR

Bump the flagsmith sdk to its latest version 3.0.0.
This most notably allows depends on the latest version of reqwest, the main motivation can be found here: Flagsmith/flagsmith-rust-client#49

Notes

There is no need to actually pull the reqwest crate ourselve for the headermap as it is a re-export of the http crate which is stable.

[coderabbitai]

📝 Walkthrough

Walkthrough

The Flagsmith SDK dependency is upgraded from version 2.1 to 3.0. The crate replaces reqwest with http for custom header types. An unused test import is removed.

Changes

Flagsmith HTTP dependency update

Layer / File(s)	Summary
Dependency and header contract crates/flagsmith/Cargo.toml, crates/flagsmith/src/lib.rs	The Flagsmith dependency is upgraded to 3.0. reqwest is replaced by http version 1. FlagsmithOptions::custom_headers now uses http::HeaderMap.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: beeme1mr

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the Flagsmith SDK dependency upgrade to version 3.0.
Description check	✅ Passed	The description explains the Flagsmith SDK upgrade and the related switch from reqwest headers to the http crate.

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[jbovet]

_{Approved in CodeRabbit Change Stack}

[jbovet]

@matthewelwell @Zaimwa9 @beeme1mr whenever you have a chance, could you please take a look at this PR? All checks are passing, and it's ready for review.