[webview_flutter_wkwebview] Fixes loadFlutterAsset exception and updates native wrapper for SecTrust and SecCertificate

[bparrishMines]

Adds support to the native wrapper to handle SecTrust and SecCertificate.

This is a part of landing #7893 by splitting of the native wrapper implementation.

Also fixes flutter/flutter#162938 and adds an integration test for loadFlutterAsset.

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[bparrishMines]

For future reference, I ran this in a terminal to generate this file

# Generate a private key
openssl genpkey -algorithm RSA -out test_key.pem -pkeyopt rsa_keygen_bits:2048

# Generate a Certificate Signing Request (CSR) - fill in dummy info
openssl req -new -key test_key.pem -out test_csr.pem

# Generate a self-signed certificate (e.g., valid for 365 days) and export to DER format
openssl x509 -req -days 5000 -in test_csr.pem -signkey test_key.pem -out test_cert.der -outform DER