Add support for Private Key Content Injection

[damithkothalawala]

Hi Team,

@ddevadat is working on an integration with OCI GO SDK where he is trying to push key contents via env variable (base64 encoded) but due to the nature of the current SDK works, it requires key file to be physically present at key location.

Could you guys please look in to
environmentProvider := environmentConfigurationProvider{EnvironmentVariablePrefix: "TF_VAR"} section and introduce a method to get this done ? And this would be helpful for our multi-cloud deployments via k8s config maps where we do not have instance-principals as a luxury.

oci-go-sdk/common/client.go

Line 334 in cbd797f

environmentProvider := environmentConfigurationProvider{EnvironmentVariablePrefix: "TF_VAR"}

Thank you in Advance.