fix(login): use verifier to create oauth challenge

Description

• Fixes oauth authentication by using the code_verifier to generate the code_challenge (previously a new verifier was used to generate the code_challenge) https://docs.gitlab.com/ee/api/oauth2.html#authorization-code-with-proof-key-for-code-exchange-pkce
• Prints an error if the OAuth request fails with a 400. Previously the user would be unaware why the request failed.

Related Issues

Resolves #7652 (closed)

How has this been tested?

glab auth login (web)

Screenshots (if appropriate):

Redacted URL since my browser isn't configured.

Before:

AFTER:

New error message:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap