feat: Support list of env files

[LeonLuttenberger]

Issue

#500

Description of changes

Add ability to specify multiple env files:

seedfarmer apply addf --env-file .env --env-file .env1 --env-file .env2

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[dgraeber]

Sooo...there are multiple places the env is used (in the CLI) - all are addressed here?

Is this backward compatible (just want to confirm)?

Did you test if ordering is preserved (I was wondering this myself)?

I suggest adding a _logger.debug to write out to the console the final resolution of all env parameters

[LeonLuttenberger]

Sooo...there are multiple places the env is used (in the CLI) - all are addressed here?

I did a search for references to --env-file in the codebase, and I changed all the ones I found. So unless there are places where the environment parameter is named differently, this should be everything

Is this backward compatible (i.e customers are passing in unbracked strings...does this work for both)?

This isn't allowing the passage of bracketed strings. Instead, it's making use of click's option to allow multiple values of a parameter. So --env-file "[.env, .env2]" won't work, but --env-file .env --env-file .env2. Another benefit of this approach is that it's automatically backwards compatible, since it doesn't require a different way for each env-file parameter to be defined.

Did you test if ordering is preserved (I was wondering this myself)?

Yes. If .env1 and .env2 are passed in that order, values from .env2 will override values from .env1 with the same key. I would like to find a more robust way of testing this though. The issue is that the test_command function execution everything in isolation, so I can't check the env variables in my unit test.

I suggest adding a _logger.debug to write out to the console the final resolution of all parameters

I like that idea.

[dgraeber]

Good work!! Also...update seed-farmer/docs/source/manifests.md near the (envVariable) section....just say that you can pass in multiple files in the order they should be resolved.