Fix instrumentation of custom cache stores

[smudge]

What does this PR do?

This fixes #3199, by avoiding the assumption that config.cache_store will be set to something that exists in the ActiveSupport::Cache:: namespace. By using the generic demodularize, it retains backwards compatibility while also supporting constants such as MyApp::CustomCache or CustomRedisCache.

Motivation:

Much like #3199, I was seeing:

    NoMethodError:
       undefined method `[]' for nil:NilClass

     # ddtrace-1.15.0/lib/datadog/tracing/contrib/active_support/cache/instrumentation.rb:104:in `dd_store_name'
     # ddtrace-1.15.0/lib/datadog/tracing/contrib/active_support/cache/instrumentation.rb:158:in `write'

How to test the change?

Install a cache store that does not exist in the ActiveSupport namespace, such as Rails' new solid_cache. Upon use, the above stack trace will be reproduced.

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.

Unsure? Have a question? Request a review!

[ekump]

Hi @smudge thank you for contributing this bug fix! The fix looks great, but would you be able to add some testing around it? The cache spec can be found here.

[smudge]

I've added two new test cases for a cache class in a different namespace and a cache class without a namespace. 👍

Tests added!

[marcotc]

Hey @smudge, this change has just been released in 1.16.0. Let us know if this addresses your issue!