Support specifying token as part of provider configuration block

[madhuravius]

To use this, add an argument for "token" to the "aptible" provider block:

provider "aptible" {
      token = "<<TOKEN>>"
}

[almathew]

We need to update the Go client before merging this. Right now it's pinned to a commit.

[madhuravius]

closing for now due to deprioritization, will return to this if/when needed