Add an HTTP format style for both Date and DateComponents (#1127)

* Add an HTTP format style for both `Date` and `DateComponents`.

* Correct the comments to properly use DateComponents type

* Use default values instead of crashing when formatting date components with missing or invalid fields

* Use 59 seconds instead of 0 for leap second of 60 - this results in being off by only 1 second in this case instead of 59.

* Address some review feedback

Author: parkera

Sources/FoundationEssentials/Calendar/Calendar.swift

@@ -681,6 +681,19 @@ public struct Calendar : Hashable, Equatable, Sendable {
         return dc
     }
 
+    /// Same as `dateComponents:from:` but uses the more efficient bitset form of ComponentSet.
+    /// Prefixed with `_` to avoid ambiguity at call site with the `Set<Component>` method.
+    internal func _dateComponents(_ components: ComponentSet, from date: Date, in timeZone: TimeZone) -> DateComponents {
+        var dc = _calendar.dateComponents(components, from: date.capped, in: timeZone)
+
+        // Fill out the Calendar field of dateComponents, if requested.
+        if components.contains(.calendar) {
+            dc.calendar = self
+        }
+
+        return dc
+    }
+
     /// Returns all the date components of a date, as if in a given time zone (instead of the `Calendar` time zone).
     ///
     /// The time zone overrides the time zone of the `Calendar` for the purposes of this calculation.

Sources/FoundationEssentials/Calendar/Date+FormatStyle.swift

@@ -55,3 +55,29 @@ extension Date {
     }
 #endif // FOUNDATION_FRAMEWORK
 }
+
+@available(FoundationPreview 6.2, *)
+extension DateComponents {
+    /// Converts `self` to its textual representation.
+    /// - Parameter format: The format for formatting `self`.
+    /// - Returns: A representation of `self` using the given `format`. The type of the representation is specified by `FormatStyle.FormatOutput`.
+    public func formatted<F: FormatStyle>(_ format: F) -> F.FormatOutput where F.FormatInput == DateComponents {
+        format.format(self)
+    }
+    
+    // Parsing
+    /// Creates a new `DateComponents` by parsing the given representation.
+    /// - Parameter value: A representation of a date. The type of the representation is specified by `ParseStrategy.ParseInput`.
+    /// - Parameters:
+    ///   - value: A representation of a date. The type of the representation is specified by `ParseStrategy.ParseInput`.
+    ///   - strategy: The parse strategy to parse `value` whose `ParseOutput` is `DateComponents`.
+    public init<T: ParseStrategy>(_ value: T.ParseInput, strategy: T) throws where T.ParseOutput == Self {
+        self = try strategy.parse(value)
+    }
+
+    /// Creates a new `DateComponents` by parsing the given string representation.
+    @_disfavoredOverload
+    public init<T: ParseStrategy, Value: StringProtocol>(_ value: Value, strategy: T) throws where T.ParseOutput == Self, T.ParseInput == String {
+        self = try strategy.parse(String(value))
+    }
+}