Move credentials.json to .env file

[benjaminbojko]

This should align more with best practices for storing sensitive info and dealing with test/live environments.

Need to consider possible backwards compatibility with:

• credentials.json
• Command line parameters
• launchpad.json config parameters

Also should think about whether there's a global credentials/sensitive info manager class or just one for content sources.

cc @pingevt

[benjaminbojko]

See #18

[claytercek]

Copying over the env cascading logic mentioned in #126:

It could be nice to also add an extra option to the launchpad CLI to allow selecting specific dotenv files, and perhaps cascade them. Example:

Given the following dotenv files (assuming files ending in .local are gitignored):

# .env
API_DATASET="production"
PC_NAME="foo"
HELLO="world"

# .env.local
PC_NAME="bar"

# .env.development
API_DATASET="development"

Running npx launchpad would load .env then load .env.local, resulting in

{
   "API_DATASET": "production",
   "PC_NAME": "bar",
   "HELLO": "world"
}

Running npx launchpad -e development would load .env then load .env.local then .env.development, resulting in

{
   "API_DATASET": "development",
   "PC_NAME": "bar",
   "HELLO": "world"
}