[design] Enable temporary creds for environment initialization

[efekarakus]

Problem statement

As of Copilot v0.2.0, users cannot use temporary credentials while creating an environment (#1068, #1104).
We want to support this use case, while still making it clear that the default credential provider chain is used to look up application metadata and hence should prefer long lived profiles.

Proposal

To satisfy the new requirement, we'll update the env init prompt as follows after the users input their environment name:

$ copilot env init

Name: prod-iad

These credentials will be used only for creating your environment.

Which credentials would you like to use to create prod-iad?
> Use temporary credentials
> [profile default]
> [profile test]
> [profile prod-iad]
> [profile prod-pdx]

If the user chooses a profile, then we'll continue down the existing flow and start creating the environment stack.

If user chooses to enter temporary credentials, then we will populate the defaults with values from their default credential provider.

Name: prod-iad

These credentials will be used only for creating your environment.

Credential source: temporary credentials

  What's your AWS Access Key ID?     (***************ZDQY)
  What's your AWS Secret Access Key? (***************2pA/)
  What's your AWS Session Token ?    (****************asdf)
  Which region? (us-west-2)

We'll introduce new flags to match the new prompts:

$ copilot env init --help
Creates a new environment in your application.

Usage
  copilot env init [flags]

Flags
  -h, --help             help for init
  -n, --name string      Name of the environment.
      --prod             If the environment contains production services.
      --profile string   Name of the profile. Exclusive with temporary credential flags.  # New description.
      --aws-access-key-id string       Temporary access key ID.               # New flag.
      --aws-secret-access-key string   Temporary secret access key.           # New flag.
      --aws-session-token string       Temporary session token.               # New flag.
      --region string    Name of the region to create the environment in.     # New flag.

Global Flags
  -a, --app string   Name of the application.

The new --region flag allows us to satisfy #1115.

Finally, we will update $ copilot init to use the default credential provider instead of the "default" profile for initializing your "test" environment.