Introduce a mechanism to help transition to TF 2.0.

In TF 2.0
- Eager execution will be enabled (what tf.enable_eager_execution() does today)
- tf.Variable will be backed by DT_RESOURCE tensors (what
  tf.enable_resource_variables() does today)
- Control flow constructions (tf.cond, tf.while_loop) will be represented in
  the graph using the functional operations

This commit sets up an environment variable that turns these behaviors
on by default. A follow up change will set this environment variable
before executing all unittests in the TF 2.0 build environments.

PiperOrigin-RevId: 218208216