Add usage string to applications

[agalbachicar]

Desired behavior

Applications should have a usage string providing a description of what they do, the main flags and an a sample execution.

Alternatives considered

N/A

Implementation suggestion

Use gflags::SetUsageMessage(kUsageMessage) before calling gflags::ParseCommandLineFlags(&argc, &argv, true) in the applications where kUsageMessage is a constexpr string.

Additional context

See this comment.