Instrument gems that are required after Datadog.configure

[marcotc]

What does this PR do?

When Datadog.configure tries to instrument a gem but that gem is not loaded it currently gives up.

With this PR, it now registers a hook on the Kernel.require method that will instrument the respective gem when that gem is loaded.

Motivation:

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

To do

• Test this with all combinations of integrations we support (e.g. load activesupport before rails. Load rails before activesupport. Same for typhoeus and ethon.)