reuse same account client when it found in vpc attachments

[RomanMelnyk113]

When a Transit Gateway peering attachment connects two TGWs in the same AWS account (but possibly different regions), the VPC discovery logic was skipped unless AWSCrossAccountRoleARN was configured, this caused kvisor to fall back to route CIDRs and miss actual VPC/subnet metadata for same-account peers.

In this PR following changes were done:

• Fetch the provider's own AWS account ID at initialization time using STS GetCallerIdentity
• Allow VPC discovery to proceed for same-account TGW peering attachments without requiring a cross-account role ARN
• Add a same-account fast-path that reuses the existing IRSA-credentialed EC2 client (or builds a region-scoped one) instead of attempting role assumption