az pipelines is missing commands for managing task groups

`az pipelines` can e.g. be used to export the definition of a build.

If that build definition (or a release definition) references a Task Group, you'll have a reference to the Task Group in the export, which is good of course.

But I would also like to export the definition of the Task group and there is no (sub)command to handle taskgroups.

Suggestion: just like the `build` and `release` subcommands, add a `taskgroup` subcommand (`az pipelines taskgroup definition list` and `az pipelines taskgroup definition show`

[Enter feedback here]

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#### Document Details

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* ID: 2a2123ca-5408-8f93-6f5c-93fe74684523
* Version Independent ID: 7a03fe73-da7f-9b5a-38e7-ef51d85d49fd
* Content: [az pipelines](https://docs.microsoft.com/en-us/cli/azure/ext/azure-devops/pipelines?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/ext/azure-devops/pipelines.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/ext/azure-devops/pipelines.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**