Support AWS_CA_BUNDLE for S3Getter

[Tener]

This PR adds support for AWS_CA_BUNDLE env var.

[hashicorp-cla]

All committers have signed the CLA.

[Tener]

@apparentlymart @jbardin sorry for pinging you directly, but I'm wondering if there is anything I can do to make this PR merged?

[Tener]

@apparentlymart @jbardin @crw Gentle ping. Is there anything for me to do here?

[crw]

Hi @Tener, I will see if I can find an owner for this.

[crw]

Hi @Tener, just to set expectations, development on go-getter is essentially frozen barring exceptional circumstances. I will link this issue to #467, on the chance that they can be handled together if we wind up needing to move to SDKv2. Thanks!

[Tener]

Thank you for letting me know @crw. For the context, the lack of support for AWS_CA_BUNDLE is causing issues with terraform when fetching modules from S3 in environment with TLS-terminating proxy with transient certificates, which are unsuitable to be installed in the system store.