Key Vault track 2 package stubs

[chlowell]

This creates a single distribution package, azure-security-keyvault. This is partly kicking the can but I like it as a reasonable first step that gets us working on master. I expect spinning off packages to be simpler than merging them, especially if we've shipped a preview, and azure-security-keyvault can continue as a metapackage after being decomposed.

Closes #4632

[adxsdk6]

Can one of the admins verify this patch?

[adxsdk6]

Can one of the admins verify this patch?

[codecov-io]

Codecov Report

Merging #5143 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5143   +/-   ##
=======================================
  Coverage    49.5%    49.5%           
=======================================
  Files        6274     6274           
  Lines      173320   173320           
=======================================
  Hits        85804    85804           
  Misses      87516    87516

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