fix: Support AWS CLI v2 SSO cache and CLI assume-role cache

[huseyinbabal]

Summary

Fix issue where taws doesn't recognize active sessions created by other tools (AWS CLI v2, external apps).

Closes #85

Changes

1. SSO Token Cache (sso.rs)

• Support CLI v2 format: SHA1(sso_session) for cache file name
• Support CLI v1/legacy format: SHA1(sso_start_url) as fallback
• Support both snake_case and camelCase field names via serde aliases
• Make region field optional (not all cache files have it)

2. AWS CLI Assume-Role Cache (credentials.rs)

• Read cached credentials from ~/.aws/cli/cache/
• Match cache files by comparing role ARN and account ID
• Check expiration before using cached credentials
• Fall back to performing STS AssumeRole if no valid cache found

3. Tests

• Add tests for CLI cache file matching logic
• Add tests for expired credential handling
• Add tests for non-matching role ARNs

What This Enables

taws can now use credentials cached by:

• aws sso login (CLI v2)
• aws sts get-caller-identity --profile <assume-role-profile>
• External tools that populate ~/.aws/cli/cache/

Priority Order for Role Assumption Profiles

When a profile has role_arn configured:

1. First check ~/.aws/cli/cache/ for existing valid credentials
2. If not found, perform STS AssumeRole (existing behavior)

This avoids unnecessary API calls when valid credentials already exist.