Add the ability to configure the refresh hint of the local bundle (sp…

…iffe#4400)

* spire-server: add support for configuring a static bundle refresh_hint

This allows us to move to a place where we are closer to the
recommendation of the SPIFFE spec which says that client of the trust
bundle endpoint should default to a low refresh interval to be able to
retrieve updated trust bundles in a timely manner.

Signed-off-by: Sorin Dumitru <[email protected]>
Signed-off-by: Anil Chaurasia <[email protected]>