add data sources for creating Infra credentials

[mxyng]

As a user, I want to use the Terraform Infra provider to authenticate with a Kubernetes cluster, given the cluster is registered with Infra server.

The registration process requires deploying an Infra connector to the cluster which requires access to the cluster. This limits the usefulness of Infra as a Terraform Kubernetes credential provider since it requires a two step process. This would be useful for distributed configurations where an admin installs the connector and gives individual users/groups access. Those non-admins, not having the root cluster credentials, can then use Infra to access the cluster.

Currently, credentials are created with an immutable lifetime of 5 minutes. While this is sufficient for something like kubectl where each request can potentially request a new set of credentials, Terraform will create the credential once when the Terraform configuration is applied. With a sufficiently large Terraform plan, the credential may expire partway through the plan.

Relates to #16

[log1cb0mb]

@mxyng Any updates on this, please?