Add GSI for Slack channel lookups and replace scan with query

[sinmi-hub]

Description

Made changes to Slack App logic. Slack channel look up now use GSI, allowing for faster query as opposed to scan

Type of change (Choose 1)

• Bug Fix
• New Feature: Non-new Integration Target
• New Feature: New Integration Target
• Documentation update
• Security fix

Reason for this change

This allows for more efficient use of DDB, in relation to Slack. With GSI, there is faster lookups by Slack for DynamoDB, which allows the slack integration to perform faster and reduce possible latency, which becomes only visible with scale.

Related Issue (if applicable)

Asana Link

Contributor Task List

• I have reviewed the Contributing Guideline and Code of Conduct
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• The code coverage difference report check has passed
• Security considerations have been addressed

Other: Provide a brief description or attach an architecture diagram to this PR.

How is state / caching maintained in the integration

N/A

How much do you expect this integration to cost

N/A

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT No Attribution license