Improve swift concurrency throughout the SDK

[etoledom]

We can use -strict-concurrency=complete flag to check the overall implementation:

.target(
    name: "Gravatar",
    swiftSettings: [
        .unsafeFlags(["-strict-concurrency=complete"])
    ]
),

or also

.target(
    name: "Gravatar",
    swiftSettings: [
        .enableUpcomingFeature("StrictConcurrency")
    ]
),