chore: Add input validation for private key in oauth flow

[Amplifiyer]

Description of changes

Add input validation for private key in sign in with apple oauth flow.

This changes CX a little bit.

Before -> CLI accepts just the key portion OR the key with header and footer as valid input when given without any line breaks, but no input validation since just the key cannot be reliably validated with regex.
After -> CLI only accepts the private key with header and footer as valid input when given without any line breaks. The input is validated to have the right header and footer.

The obvious benefit of this change is to remove any guesswork on CX to figure out the right format. There are at least 4 different customers who have run into this issue trying to figure out the format (e.g. providing the absolute path to the .p8 file or providing just the key but with line breaks and the prompter just takes the first line as input)

This also brings it in parity with validation in Studio

Issue #, if available

fix: #8097, #9478

Docs PR: aws-amplify/docs#5018

Description of how you validated changes

e2e tests and manually tested the walkthrough for both add and update.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.