Allow for multi-account push into AWS

[SteveMcGrath]

Findings appeared in AWS, but the script pushed all findings related to different AWS accounts to the Security Hub of the specified AWS account in the config file.

Originally posted by @tracker5676 in #43

This will likely require a configuration flag to support this in a backwards compatible way.

[SteveMcGrath]

A new optional parameter has been added to the config file called map_to_asset_account is now available that will attempt to fetch the aws account id from the TVM-stored asset and use that instead. It will still fall back to using the configured account if it cannot be found, or if the config setting is set to false.

To enable, simply set this flag to true