`copilot env init` should fall back to "default" profile in absence of AWS profiles

[Lou1415926]

When running copilot env init, Copilot will:

1. Read the AWS profiles from the machine (usually, this is a file in the ~/.aws/credentials path);
2. Prompt the user to select a profile with which the environment should be created with.

Today, if Copilot fails to read the profiles in step 1, it simply errorr out. This requires users to: 1. Either configure a profile, OR 2. use temporary credentials

For a user that is not familiar with AWS, this is confusing; for a toned AWS user that doesn't use shared profiles, this is not convenient.

My proposal is that Copilot should read the AWS profiles from the machine (usually, this is a file in the ~/.aws/credentials path), then:

1. If the profile is not configured (i.e. the profile file - usually ~/.aws/credentials - is absent), then Copilot defaults to whatever aws sts get-caller-identity returns. In this case, the env session would use

   copilot-cli/internal/pkg/aws/sessions/sessions.go

   Line 83 in 3adda09

   func (p *Provider) Default() (*session.Session, error) {

2. If it fails to read the profiles for other reasons, error out
3. If it is able to read the profile, prompt.

[Lou1415926]

For anyone that comes across this - this enhancement is already released in AWS Copilot v1.31.0! For more, see the release note and the blog post 🎉🚀 🚀