Geo: Fix Dynamic Backoff strategy and add specs

What does this MR do?

Moves linear_backoff_strategy and geometric_backoff_strategy to class methods in the GeoDynamicBackoff module so a include will work as initially expected.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

In a previous version when we refactored the backoff strategy we actually broke it in the process.

See #2639 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2639 (closed)

cc @stanhu @dbalexandre @rspeicher