Source environment variables from `.env` file when adding deployment secrets

Currently, setting secrets is a very manual process and can be streamlined using a users `.env` file. The `.env` file can be referenced in the command line so that they get sourced and uploaded as secrets.

```
$ krane secrets add my-deployment -f ./.env
```

If an environment variable defined in the `.env` ALREADY exists it should not be overridden

```
Done uploading secrets for my-deployment sourced from ./env:
DB_URL: @DB_URL
DB_USER: @DB_USER

We found some secrets which where already in use and not overriden:
DB_PASSWORD: @DB_PASSWORD
```

Start at the `krane secrets add` CLI entry point and inspect how deployment secrets are currently collected and uploaded. Use the `.env` example in the issue to define the expected invocation and output; done means variables are sourced from the file and existing secrets are reported without being overridden.

Written by the indexing model from the issue text.