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
• [x] 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 :white_check_mark:

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

:exclamation: 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

```diff @@ 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 ```

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