Support aws login

[khmoryz]

Upgrade to aws-sdk-go-v2 v1.40.0 (latest)

This change adds support for the aws login command, which simplifies acquiring AWS credentials.

Reference:
https://aws.amazon.com/jp/blogs/security/simplified-developer-access-to-aws-with-aws-login/

Quick test

❯ ./rain ls
could not establish AWS credentials; please run 'aws configure' or choose a profile

❯ aws login
Attempting to open your default browser.
If the browser does not open, open the following URL:

...

❯ ./rain ls
CloudFormation stacks in ap-northeast-1:

...

❯ aws logout
Removed cached login credentials for profile 'default'. Note, any local developer tools that have already loaded the access token may continue to use it until its expiration. Access tokens expire in 15 minutes.

❯ ./rain ls 
could not establish AWS credentials; please run 'aws configure' or choose a profile