Enable release mode builds (#69)

### Motivation:

Some errors do not show up in debug builds. Enabling release mode builds
improves the CI coverage.

### Modifications:

Enable release mode builds for pull requests and scheduled builds on
main.

### Result:

Improved CI coverage.

Author: josephnoir

.github/workflows/main.yml

@@ -29,3 +29,7 @@ jobs:
             build_scheme: swift-http-structured-headers-Package
             xcode_16_2_build_arguments_override: "-Xswiftc -Xfrontend -Xswiftc -require-explicit-sendable"
             xcode_16_3_build_arguments_override: "-Xswiftc -Xfrontend -Xswiftc -require-explicit-sendable"
+
+    release-builds:
+        name: Release builds
+        uses: apple/swift-nio/.github/workflows/release_builds.yml@main

.github/workflows/pull_request.yml

@@ -36,3 +36,7 @@ jobs:
             build_scheme: swift-http-structured-headers-Package
             xcode_16_2_build_arguments_override: "-Xswiftc -Xfrontend -Xswiftc -require-explicit-sendable"
             xcode_16_3_build_arguments_override: "-Xswiftc -Xfrontend -Xswiftc -require-explicit-sendable"
+
+    release-builds:
+        name: Release builds
+        uses: apple/swift-nio/.github/workflows/release_builds.yml@main