Added require-explicit-sendable to macOS CI. (#39)

Added require-explicit-sendable to macOS CI

### Motivation:

Make macOS CI fail in case we have public types not explicitly marked as
Sendable or explicitly marked as non sendable.

### Modifications:

Enabled extra flags to macOS CI build.

### Result:

Swift compiler will check for public types without an explicit Sendable
statement.

Author: rafaelcepeda

.github/workflows/main.yml

@@ -32,3 +32,5 @@ jobs:
     with:
       runner_pool: nightly
       build_scheme: swift-nio-oblivious-http-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"

.github/workflows/pull_request.yml

@@ -36,3 +36,5 @@ jobs:
     with:
       runner_pool: general
       build_scheme: swift-nio-oblivious-http-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"