Add a prompt to 'aws login' to warn users when updating a profile with existing credentials

[ashovlin]

Issue #, if available: #9869

Description of changes:

This adds a prompt to aws login to warn users when running it for a profile that already has a different style of credentials:

$ aws login

Warning: Profile 'default' is already configured with Access Key credentials. If you continue to log in, the CLI and other tools may continue to use the existing credentials instead.

You may run 'aws login --profile new-profile-name' to create a new profile, or else you may manually remove the existing credentials from 'default'.

Do you want to continue adding login credentials to 'default'? (y/n):

This should help avoid the confusing case where you run aws login but subsequent commands are still using different credentials.

I did not add support for clearing the other style of credentials yet. For access keys there wouldn't be an easy way to undo or retrieve them if we just delete them from the credentials file, so I'd want to be more careful here. If we get feedback, we could expand from y | n to y | n | clear in the future.

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[hssyoo]

This seems like a weird experience and I feel like we should be gatekeeping here. Are there reasons we shouldn't just deny the login attempt here?