[Feature Request] Ability to query username for currently logged in user

**Is your feature request related to a problem? Please describe.**
I would like to filter resources for the *currently logged in* user within a JMESpath `--query` (without knowing the username). So in order to pass the username into the query, I would need a way to retrieve the login name of the currently logged in user first.

**Describe the solution you'd like**
For example: `az devops account show` should return login context information of the currently logged in user, including his username.

**Additional context**
Note: I tried `az account show`, but that errors and tells me:
> Please run 'az login' to setup account.