[CT-1869] Turn --output-keys into MultiOption

[jtcohen6]

Current state

Users can supply --output-keys to customize the set of information returned by dbt list:

$ dbt ls --output json --output-keys name,resource_type,description
{"name": "my_incremental_model", "resource_type": "model", "description": "cool docs bro"}
{"name": "my_seed", "resource_type": "seed", "description": ""}

The type of --output-keys is a string. I've always thought the string needed to be a comma-delimited list, but really it could be ... anything? ... so long as the string contains the relevant key name:

dbt-core/core/dbt/cli/params.py

Lines 157 to 159 in c85be32

	output_keys = click.option(
	"--output-keys", envvar=None, help="TODO: No current help text", type=click.STRING
	)

$ dbt ls --output json --output-keys nameNotACommaresource_type==blabladescription
{"name": "my_incremental_model", "resource_type": "model", "description": "cool docs bro"}
{"name": "my_seed", "resource_type": "seed", "description": ""}

Proposal

We just (#6641) added a MultiOption option to support params like --select, that take multiple space-limited arguments:

$ dbt run --select thing_a thing_b --another-flag ...

This would be more consistent, and allow us to treat the type of output_keys as a list (right?) for programmatic invocations:

$ dbt ls --output json --output-keys name resource_type description

Looks like that's even how we've been documenting it (incorrectly)! https://docs.getdbt.com/reference/commands/list

Add a test

We also need an automated test for dbt list --output json --output-keys!