feat(transform): Add Meta KV Store Lock Transform

[jshlbrd]

Description

• Adds meta_kv_store_lock transform
  • Adds Locker interface to internal/kv
  • Adds Locker support to DynamoDB and Memory KV stores
• Adds "exactly once" examples to docs
• Refactors enrich_kv_store_set transform

Motivation and Context

This adds the ability for any message to become "locked" in a KV store (typically, a distributed database) and have a transform applied to it. This addresses #59 with some additions and caveats:

• This is not idempotency, but idempotency is possible by combining this transform with a few others
• Any transform function can make use of this feature, not just "producers" (this concept no longer exists in Substation v1.0+)

How Has This Been Tested?

Integration tested using the added examples. Failure scenarios can be tested using the DynamoDB distributed lock example.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.