What does this PR do?

Enhancement:

• use context propagation in functions instead of using a struct field. This allows the caller to control the lifecycle of the functions (and set special variables on the context, for instance context.WithValue(ctx, datadog.ContextServerIndex, 2) which disable the check preventing from trying to contact staging sites
• add a secret string structure. Before, app and api keys were set in the context and propagated everywhere. Logging the context by inadvertance would leak the credentials. Now, it will be redacted. Values are still visible in memory, but they already were before

Motivation

• Control execution lifecycle through context
• Propagate tagging & info in context