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: "plaintext", "tls", "reloadingTLS", "mTLS", "reloadingMTLS") with a "transportSecurity.mode" key (3 values: "plaintext", "tls", "mTLS").
• Added a "transportSecurity.credentialSource" key ("inline", "file"), and a "transportSecurity.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.

[czechboy0]

A few suggestions, otherwise lgtm

[czechboy0]

Is this right? The input to the init(config:) of SupportedHTTPVersions is scoped to http only, the versions is then parsed internally and that type is HTTPVersionKind instead.

[aryan-25]

Yes, sorry, SupportedHTTPVersions was a remnant from working on #67, but we eventually settled on using Set<HTTPVersion> to represent supported HTTP versions instead. I've updated the docs.

[czechboy0]

Suggested change

	/// - `gracefulShutdown.maximumGracefulShutdownDuration` (int, optional, default: nil): The maximum amount of time
	/// - `gracefulShutdown.maximumDuration` (int, optional, default: nil): The maximum amount of time

Suggestion

[aryan-25]

Agreed.

I realised we also have a maximumGracefulShutdownDuration property under the GracefulShutdownConfiguration type in NIOHTTPServerConfiguration:

swift-http-server/Sources/NIOHTTPServer/Configuration/NIOHTTPServerConfiguration.swift

Line 143 in b243999

public var maximumGracefulShutdownDuration: Duration?

I'm going to address that in a separate PR where we will also make the other NIOHTTP2ServerConnectionManagementHandler options configurable; currently we only set the maxGraceTime option:

swift-http-server/Sources/NIOHTTPServer/NIOHTTPServer+SecureUpgrade.swift

Lines 147 to 151 in b243999

	maxIdleTime: nil,
	maxAge: nil,
	maxGraceTime: configuration.gracefulShutdown.maximumGracefulShutdownDuration
	.map { TimeAmount($0) },
	keepalive: nil

[czechboy0]

This table is a really good idea, I'll steal it for other projects too :)