Add `invoke` command to run a plugin in isolation

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Add tests for the proposed code
• Include a README.md change

Once your merge request is complete, please assign it a Meltano maintainer for review cycle. Once the review cycle finished, the reviewer shall approve the merge request so it can be merged by any Meltano maintainer.

This MR adds the meltano invoke command to run a plugin manually. Meltano will inject the default parameters by defaults.

Tap:

• --config
• --catalog if a catalog is found
• --state if a state is found

Target:

• --config

This command is particularly helpful to test/debug plugins.