Add env as first-class attribute of MeltanoFile and ProjectPlugin classes (not an extra)

[kgpayne]

Note: env is already a first-class attribute of the Environment class.
Note: Just adding these will not make inheritance work (as per #5982 and #5983).

Spec:

• env is a first-class attribute of MeltanoFile accessible via top-level env: key in meltano.yml)
• env is a first-class attribute of ProjectPlugin accessible via plugin-scoped env: keys in base and environment-specific config
• env is settable via the CLI, including with the '.' syntax (e.g. meltano --no-environment config meltano set env.MY_ENV_VAR value)
• New attributes are added to jsonschema, tested and documented

Release:

As both #5982 and #5983 depend on an env attribute, they should include these changes in their PR to master. This issue will be closed when either of those issues is merged to main.

[aaronsteers]

@kgpayne - We've closed #5969 in favor of this issue. Can you include the docs as part of this?

(I'm a fan of "docs driven development", so writing docs first might not be a bad idea. 😉 )

[z3z1ma]

@kgpayne Is this the idea?

just putting some lame pseudo-code yaml to illustrate the question

env:
  DATABASE: COOL_DB
  DB_PASS: ${FROM_SHELL_ENV_VAR}
  
plugin:
  transformer:
    config:
      database: ${DATABASE}
      password: ${DB_PASS}
  loader:
    config:
      target_db: ${DATABASE}
      postgres_pass: ${DB_PASS}

[tayloramurphy]

@z3z1ma yes and you'd be able to set env at the plugin level as well. This is basically taking https://docs.meltano.com/concepts/environments#the-env-mapping and bringing it to all levels of your meltano.yml.