PXD-1287 ⁃ add pytest fixtures for generating RSA key strings

[rudyardrichter]

Several other CDIS repos need to load or generate RSA keys for running tests. To avoid duplicating code, it would be useful for authutils to export pytest fixtures which return RSA public and private keys.

TODO

• add pytest fixtures
• update other services to use the new fixtures:
  • fence
  • sheepdog
  • peregrine