[Foundation] NSUrlSessionHandler: Adds support for X509 client certificates

[rolfbjarne]

Addresses #13856

This was originally created by @dotMorten in #20434.

Also make SecIdentity.Import use an in-memory keychain on macOS 15+, so that
SecIdentity.Import works like all othe other platforms (i.e. not requiring
access to the default keychain, which, among other things, is not ideal on
bots).

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Big Sur (11) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11) passed.

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

❌ [CI Build] Windows Integration Tests failed ❌

❌ Failed ❌

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]

[dotMorten]

Ship it! :-) 🚢

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 97 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 1 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 40 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 7 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 7 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 7 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: 3be26ada7d7177f696d401e8651042aa87969453 [PR build]