feat!: make DescriptorExtension internal

[sajayantony]

What this PR does / why we need it

Need to consider designing this API if we want to expose this.

Which issue(s) this PR resolves / fixes

Fixes #233

Please check the following list

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.29%. Comparing base (bc29f72) to head (5973e20).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #239   +/-   ##
=======================================
  Coverage   89.29%   89.29%           
=======================================
  Files          52       52           
  Lines        2280     2280           
  Branches      305      305           
=======================================
  Hits         2036     2036           
  Misses        161      161           
  Partials       83       83           

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[shizhMSFT]

LGTM