Add missing user object member to type BlockAction

[GovernmentHack]

Summary

Adds missing string type member to user object in BlockAction type. Fixes (issue 1909)[https://github.com//issues/1909]

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @gapodaca-atlassian to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thank you!

[filmaj]

This PR was helpful but we need to go a bit farther: the name field should now be optional (based on the block_actions documentation, Home tabs might return block_action payloads with a name field). I also think the unit tests are failing due to this addition.

Will try to fix this up and send another PR.