feat: Support role_arn and source_profile for IAM role assumption

[huseyinbabal]

Summary

Add support for assuming IAM roles using role_arn and source_profile configuration in ~/.aws/config. This is a common pattern in enterprise environments for cross-account access and least-privilege security.

Closes #13

Changes

• Parse config options: role_arn, source_profile, external_id, role_session_name, duration_seconds, region
• Recursive resolution: source_profile can point to another profile that also uses role assumption (chained roles)
• STS AssumeRole: Call STS API with SigV4 signed HTTP requests
• Credential caching: Cache temporary credentials with automatic refresh before expiration
• Credential chain: Integrated at priority 3 (after SSO, before credentials file)

Example Configuration

[profile base]
region = us-east-1

[profile production]
role_arn = arn:aws:iam::123456789012:role/ProductionAccess
source_profile = base
region = us-west-2

[profile partner-account]
role_arn = arn:aws:iam::987654321098:role/PartnerAccess
source_profile = base
external_id = my-external-id

Supported Options

Option	Required	Description
role_arn	Yes	ARN of the IAM role to assume
source_profile	Yes	Profile to use for source credentials
external_id	No	External ID for cross-account trust policies
role_session_name	No	Custom session name (default: taws-session)
duration_seconds	No	Session duration in seconds (default: 3600)
region	No	Region for STS endpoint

Testing

• Added unit tests for XML parsing, error handling, and caching
• All 64 tests pass
• Builds successfully on release profile