Improve SwiftConfiguration integration

[aryan-25]

Motivation:

PR #67 refactored NIOHTTPServerConfiguration. As part of this PR, we introduced support for TLS credentials and mTLS trust roots to be provided as PEM files too.

We should update the swift-configuration integration to match these changes. We should also use this opportunity to better document the possible keys and supported values in a DocC article, as the current documentation is spread inline across multiple types.

Modifications:

• Replaced the "security" key (5 values) with a "mode" key (3 values: "plaintext", "tls", "mTLS").
• Added a credentialSource key ("inline", "file"), and a "trustRootsSource" key ("inline", "file", "systemDefaults", "customCertificateVerificationCallback").
• Renamed/rescoped several configuration keys for clarity ("supportedHTTPVersions" → "http.versions", "low"/"high" → "lowWatermark"/"highWatermark", scoped "gracefulShutdown").
• Added a DocC article with a complete configuration key reference table, example JSON, and narrative documentation.

Result:

Improved swift-configuration integration.