Make HTTPHeaders Sendable (#3856)

### Goals ⚽
Similar to `HTTPMethod` in #3848, `HTTPHeaders` is trivially `Sendable`,
so add the conformance (and others).

### Implementation Details 🚧
Explicitly declared conformances.

### Testing Details 🔍
No additional tests, conformances are synthesized.

.github/workflows/ci.yml

@@ -265,7 +265,7 @@ jobs:
  - name: Install Firewalk
  run: brew install alamofire/alamofire/firewalk || brew upgrade alamofire/alamofire/firewalk xcbeautify && firewalk &
  - name: Test SPM
- run: swift test -c debug | ${{ matrix.outputFilter }}
+ run: swift test -c debug 2>&1 | ${{ matrix.outputFilter }}
  Linux:
  name: Linux
  runs-on: ubuntu-latest
@@ -283,6 +283,11 @@ jobs:
  - image: swift:5.9-centos7
  - image: swift:5.9-amazonlinux2
  - image: swift:5.9-rhel-ubi9
+ - image: swift:5.10-focal
+ - image: swift:5.10-jammy
+ - image: swift:5.10-centos7
+ - image: swift:5.10-amazonlinux2
+ - image: swift:5.10-rhel-ubi9
  - image: swiftlang/swift:nightly-focal
  - image: swiftlang/swift:nightly-jammy
  - image: swiftlang/swift:nightly-amazonlinux2
@@ -315,6 +320,9 @@ jobs:
  - branch: swift-5.9-release
  tag: 5.9-RELEASE
  name: Windows Swift 5.9
+ - branch: swift-5.10-release
+ tag: 5.10-RELEASE
+ name: Windows Swift 5.10
  steps:
  - name: Setup
  uses: compnerd/gha-setup-swift@main

Source/Core/HTTPHeaders.swift

@@ -25,7 +25,7 @@
 import Foundation
 
 /// An order-preserving and case-insensitive representation of HTTP headers.
-public struct HTTPHeaders {
+public struct HTTPHeaders: Equatable, Hashable, Sendable {
  private var headers: [HTTPHeader] = []
 
  /// Creates an empty instance.
@@ -185,7 +185,7 @@ extension HTTPHeaders: CustomStringConvertible {
 // MARK: - HTTPHeader
 
 /// A representation of a single HTTP header's name / value pair.
-public struct HTTPHeader: Hashable {
+public struct HTTPHeader: Equatable, Hashable, Sendable {
  /// Name of the header.
  public let name: String