Add support for AWS STS tokens

[krisiye]

• Adds a new enum for supported AWS credential types. Defaults to iam_user
  • Supports Vault role that allows a single AWS role ARN. (Multiple role/arn associations will require role/arn to assume be a configurable parameter in the future for assumed_role)
• Updates to VaultAwsProperties to include credential type as well as session token for AWS STS.
• Add support for ttl for assumed_role or federation_token
• Add support for role_arn for assumed_role for vault roles that have more than one IAM role associated.

Fixes #572

Note: spring cloud AWS does not support STS session tokens yet. 
However, this enhancement will allow us to get an STS-backed session token as well as use spring cloud aws for the accessKey and secretKey.  

See https://github.com/awspring/spring-cloud-aws/issues/73

[krisiye]

@mp911de - When you have a chance can you please review this PR? Thanks for your help!

[mp911de]

Can you please rebase the PR? Merge commits sometimes cause trouble when merging pull requests.

[krisiye]

Can you please rebase the PR? Merge commits sometimes cause trouble when merging pull requests.

Done

[mp911de]

Thank you for your contribution. That's merged and polished now.