ref(options): Delegate deserialization to serde instead of generating it per-field

[jan-auer]

The SentryOptions derive macro previously generated a deserialize method that called options.get() for each struct field individually. This meant the proc macro owned all deserialization logic, including a custom #[sentry_options(rename)] attribute for field renames.

Deserialization is now a provided default method on the SentryOptions trait: it parses the embedded schema to discover property keys, fetches each value, and delegates to serde_json::from_value for the actual struct construction. Options structs must derive serde::Deserialize, and field renames use standard #[serde(rename)].

The derive macro is reduced to emitting only the two constants (NAMESPACE, SCHEMA) and the singleton/refresh glue.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.58%. Comparing base (d49434e) to head (88d9b13).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
objectstore-options/src/lib.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
+ Coverage   87.41%   87.58%   +0.16%     
==========================================
  Files          92       93       +1     
  Lines       14695    14819     +124     
==========================================
+ Hits        12846    12979     +133     
+ Misses       1849     1840       -9     

Components	Coverage Δ
Rust Backend	92.18% <ø> (ø)
Rust Client	79.89% <ø> (ø)
Python Client	88.87% <ø> (ø)

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