Add support for local metadata

[jhorwit2]

It'd be nice if we exposed a way to call the local metadata service since we use this across most of the go projects I'm aware of that use the go client.

https://docs.cloud.oracle.com/iaas/Content/Compute/Tasks/gettingmetadata.htm?tocpath=Services%7CCompute%7C_____17#AccessingInstanceMetadataonOracleProvidedImages

[jodoglevy]

Thanks @jhorwit2, we'll consider adding this. The local metadata service is an unauthenticated endpoint I think, so it should be pretty easy to make requests to even without first class support in the Go SDK.

[jhorwit2]

https://github.com/oracle/oci-flexvolume-driver/tree/master/pkg/oci/instancemeta https://github.com/oracle/oci-volume-provisioner/tree/master/pkg/oci/instancemeta https://github.com/oracle/oci-cloud-controller-manager/tree/master/pkg/oci/instancemeta

Are all projects that currently re-implement this call (with more internal).