NEW Enable storing graphql schema in `silverstripe-cache/`

[GuySartorelli]

A note about testing

The acceptance criteria for testing this was originally written with the idea that this would be the default behaviour, which is no longer the plan.

Note that while I have tested this in both SC and SCPS, all I can verify is that there weren't any obvious problems from a user perspective. I have no way to validate what was happening from a filesystem POV in that environment.

Issue

• https://github.com/silverstripe/silverstripe-graphql/issues/529

[emteknetnz]

Tested locally, works good

[emteknetnz]

Opps accidentally closed it - I somehow mistook the close button for the merge button :-)