Subcommands --output enhancement for plain text

[KazChe]

Description

Part of enhancements for issue #87. It only adds support for text output format to subcommands that already support --output flag. This provides a plain text output that's easier for scripting/parsing.

Subcommands Enhanced

• tenant list/get
• instance list/get
• customer-managed-key list/get
• data-api graphql commands list/get (if enabled)

[KazChe]

I think there are few requirements that needs to be hashed out before proceeding further. So, closing this PR for now.

1. What is the expected format for plain text output?

• Right now I've implemented tab separated values
• Should it be consistent across all commands or command-specific?
• and should it include all fields from JSON output or a subset?

Appreciate any guidance/clarification on these points to make sure the implementation meets the intended requirement.