Feat: Allow plugins to declare a default command

[aaronsteers]

Currently if there is no command specified in meltano run or meltano invoke, meltano will use the executable at the plugin level.

Context

For the new plugin SDK, we want to be able to have a passthrough so that meltano invoke my-dbt run "just works" even if the actual syntax is my-plugin invoke run.

We want meltano to run this:

my-plugin invoke run

As a result of us executing this:

meltano invoke my-plugin run

Disambiguation / Pre-Analysis Needed

We should test if a viable alternative would be to have the plugin declare top-level args such as args: invoke, and if that would solve this usage pattern without needing a new feature.

[aaronsteers]

@pandemicsyn - This is a follow-up to our conversation. What I don't know is whether we can mitigate by just declaring a default arg of invoke or if that has some other issues which would prevent it from being viable.

[pandemicsyn]

👍 This seems like it could be a nice UX shortcut to have available, in general. Even without considering what we need on the SDK (which this would also help clean up).

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