Implement NewSubdirectoryClient on *directory.Client

[adreed-msft]

• The purpose of this PR is explained in this or a referenced issue.
• The PR does not update generated files.
  • These files are managed by the codegen framework at Azure/autorest.go.
• Tests are included and/or updated for code changes.
• Updates to module CHANGELOG.md are included.
• MIT license headers are included in each file.

[vibhansa-msft]

We need to regenerate the API view and get approval for this.

[github-actions]

Hi @adreed-msft. Thank you for your interest in helping to improve the Azure SDK experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[github-actions]

Hi @adreed-msft. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing /reopen if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the no-recent-activity label; otherwise, this is likely to be closed again with the next cleanup pass.

[tanyasethi-msft]

Adding changes done in this PR to #22177 to be a part of STG78-82 release of AzDatalake