Interface for providing encryption materials.
Implementations are free to use any strategy for providing encryption
materials, such as simply providing static material that doesn't change,
or more complicated implementations, such as integrating with existing
key management systems.
Forces this encryption materials provider to refresh its encryption
material. For many implementations of encryption materials provider,
this may simply be a no-op, such as any encryption materials provider
implementation that vends static/non-changing encryption material.
For other implementations that vend different encryption material
throughout their lifetime, this method should force the encryption
materials provider to refresh its encryption material.