feat: add workspace add-user command

[eshwarhs]

Description

This PR adds a new CLI command python -m argilla workspaces create.

Closes #3680

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• New feature (non-breaking change which adds functionality)

How Has This Been Tested

Unit tests have been added to tests/unit/tasks/workspaces directory

• Add user to workspace
• Add user to non-existent workspace
• Add non-existent user to workspace
• Add user belonging to same workspace
• Add user to workspace without specifying workspace name
• Add user to workspace without logging in

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
src/argilla/tasks/workspaces/__main__.py	100.00%
src/argilla/tasks/workspaces/add_user.py	100.00%

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[gabrielmbmb]

Hey @eshwarhs, thanks for the PR! Just one comment :)