Support linking against ada via feature

[logan-gatlin]

Summary

Adds the ability to skip compiling the bundled C++ ada sources and instead link against an externally provided ada library. This is useful when ada is already provided by the host build system (e.g. Bazel, CMake, or a distro package), where compiling the bundled copy would otherwise produce a second, duplicate copy of ada in the final binary.

Changes

• New bundled feature (Cargo.toml): Added to the default feature set. Compiles and statically links the bundled C++ ada sources. Disable via default-features = false to link an external ada instead.
• build.rs: When bundled is disabled, the build script skips the bundled compile and emits link directives driven by two optional environment variables:
• ADA_LIB_DIR — directory added to the linker search path (cargo:rustc-link-search=native).
• ADA_LIB_NAME — library to link, passed verbatim to cargo:rustc-link-lib (e.g. ada or static=ada).
• If neither is set, no link directive is emitted, leaving the host build system to provide the ada symbols at final link time.
• README.md: Documents the new bundled feature and a "Linking against an external ada" section with usage examples.

Notes

• The external ada must expose the same C ABI (ada_*) and be ABI-compatible with this crate's version.
• Disabling default features also drops std, so consumers should re-enable it explicitly:
  ada-url = { version = "3", default-features = false, features = ["std"] }

[anonrig]

Hi Logan!

Is there a way to test this, so we don't regress?

[anonrig]

Also can you update the README.md with this change?

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

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_{Comparing logan-gatlin:main (cca7eb5) with main (14b918f)}

[logan-gatlin]

Hi Yagiz, I updated the existing feature powerset to always use bundled, and put the unbundled tests in their own job. I also updated the README