refactor: add IRSA as first precedent for aws client creds

[prestwich]

Motivation

Enable usage of a standard creds mechanism. We thought that this was supported by the default credentials provider, but it is not. This was an oversight in our reqs and verification

Solution

• break global aws client state out into a new nomad_core::aws module
• add get_client method to instantiate a shared aws Client
• add get_kms_client method to instantiate a shared KmsClient
• add WebIdentityProvider as preferred credentials provider for Client instantiation
• fall back to DefaultCredentialsProvider if WebIdentityProvider fails to return credentials
• modify processor to use the global Client to instantiate its S3Client

PR Checklist

• Added Tests
• Updated Documentation
• Updated CHANGELOG.md for the appropriate package

[luketchang]

Approving contingent on https://github.com/nomad-xyz/rust/pull/202/files#r900469033 :)