feat(assume): add --export-as option

[QuentinBtd]

Description

This PR implements the feature requested in #821 by adding support for exporting credentials to ~/.aws/credentials under a custom profile name.

What changed

• Added a new assume flag: --export-as

  • Example: assume <source-profile> --export-as <new-profile-name>

• Updated export flow to:

  • Trigger credential export when --export-as is provided (even without --export)
  • Use the provided custom profile name as-is

• Extended credential export implementation with:

  • ExportCredsToProfileWithOptions(profileName, creds, applySuffix bool)

Behavior details

• Existing behavior is unchanged for --export:
  • ExportCredentialSuffix is still applied when configured.
• New behavior for --export-as:
  • ExportCredentialSuffix is intentionally not applied.
  • The destination profile name is exactly the value passed to --export-as.

Closes #821.