ADT UAI Extra Updates

[vilit1]

Goals of this pr:

1. add UAI for data history
2. fix some nit wordings in help from previous UAI pr
3. try to make the deletion of UAI's smoother by disassociating them from the dt's before deleting them

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[vilit1]

https://dev.azure.com/azureiotdevxp/aziotcli/_build/results?buildId=7190&view=results

[vilit1]

Do we need a history update for this? It looks like we added at least one param (user_identity)

I will add one when it is closer to actually being merged in (I hate merge conflicts with history)

Here is the blurb that I will put:

**Digital Twins**

 * Addition of User Assigned Identities for data history connections. The command `az dt data-history connection create adx` now can take an extra parameter `--mi-user-assigned` to use an associated User Assigned Identity for the connection creation rather than the system assigned identity for the Digital Twin.
 * Some minor improvements to command documentation involving managed identities.

[vilit1]

https://dev.azure.com/azureiotdevxp/aziotcli/_build/results?buildId=7211&view=results