Add java.time.InstantSource

`java.time.Clock` is an abstract class representing "now" in java.time. It has two distinct aspects - obtaining the current instant and providing the associated time zone. User feedback and examination of other libraries indicates a desire for an interface focussed only on the task of obtaining the current instant.

See the linked CSR for the proposed spec.

The existing `Clock` abstract class would be altered to implement the interface.

https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-May/077213.html