[CT-3177] [CLI] Raise deprecation warning for legacy MultiOption syntax

[jtcohen6]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Thanks to #8598, dbt-core's "multi-option" parameters support three ways of passing in arguments:

$ dbt --option arg1 arg2
$ dbt --option "arg1 arg2"
$ dbt --option arg1 --option arg2

The first is legacy behavior, and not standard across most CLIs. We should officially deprecate this behavior, and raise a warning encouraging the user to opt for (2) or (3) instead. Probably (2) is the simplest & quickest way to migrate from (1).

---

This seems to be a potential spot:

                    if not done:
                        value_list.append(state.rargs.pop(0))
                        print("Legacy multi-option support is deprecated. Use quotes instead.")

$ dbt --quiet list --select one two
Legacy multi-option support is deprecated.
$ dbt --quiet list --select "one two"
$ dbt --quiet list --select one --select two

Technical trickiness:

• This is almost certainly being resolved before we have a logger / event manager
• If we want to provide the most helpful warning message possible, we should wait until we've collected all arguments, and then reconstruct the same thing with quotes

Describe alternatives you've considered

Not adding this deprecation warnings, allowing users to stub their toes when they encounter inconsistencies across CLIs / OSes

Who will this benefit?

Users of multiple dbt CLIs

Are you interested in contributing this feature?

No response

Anything else?

• [CT-3095] [Bug] MultiOption CLI option should accept quoted strings with multiple options #8598
• Update recommended syntax for --select with unions + intersections on CLI docs.getdbt.com#4040