chore: rename Agent to Extension

[duncanista]

What does this PR do?

Renames agent to extension in extension.ts and some other files.

Motivation

Just because.

Testing Guidelines

N/A

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.95%. Comparing base (3ae6bc3) to head (2decf3f).

❗ Current head 2decf3f differs from pull request most recent head 4ed69aa. Consider uploading reports for the commit 4ed69aa to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #520   +/-   ##
=======================================
  Coverage   81.95%   81.95%           
=======================================
  Files          54       54           
  Lines        2206     2206           
  Branches      512      512           
=======================================
  Hits         1808     1808           
  Misses        334      334           
  Partials       64       64           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.