Set the default value of addExtension to true

[jcstorms1]

What does this PR do?

Enables the extension by default. Sets addExtension env var to true.

Motivation

To move towards using the extension for AWS Lambda, rather than the forwarder.

Testing Guidelines

Unit and Integration testing

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

[IvanTopolcic]

LGTM 👍

[nhinsch]

This looks good, let's do a major version bump of the plugin (e.g. 4.0) since this is a breaking change.

[codecov-commenter]

Codecov Report

Merging #233 (6b44ddb) into master (eac2507) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
- Coverage   76.51%   76.39%   -0.12%     
==========================================
  Files          19       19              
  Lines         894      894              
  Branches      215      215              
==========================================
- Hits          684      683       -1     
- Misses        181      182       +1     
  Partials       29       29              

Impacted Files	Coverage Δ
src/env.ts	93.25% <ø> (ø)
src/index.ts	69.93% <0.00%> (-0.62%)	⬇️

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