[Satosa] Example pyeudiw_backend.yaml can be secured more about private keys configuration

[Zicchio]

The example configuration file of the satosa backend, which is https://github.com/italia/eudi-wallet-it-python/blob/dev/example/satosa/pyeudiw_backend.yaml includes some hardcoded private keys.
For example, see this one.

eudi-wallet-it-python/example/satosa/pyeudiw_backend.yaml

Lines 89 to 96 in ac69db4

	federation_jwks:
	- kty: RSA
	d: QUZsh1NqvpueootsdSjFQz-BUvxwd3Qnzm5qNb-WeOsvt3rWMEv0Q8CZrla2tndHTJhwioo1U4NuQey7znijhZ177bUwPPxSW1r68dEnL2U74nKwwoYeeMdEXnUfZSPxzs7nY6b7vtyCoA-AjiVYFOlgKNAItspv1HxeyGCLhLYhKvS_YoTdAeLuegETU5D6K1xGQIuw0nS13Icjz79Y8jC10TX4FdZwdX-NmuIEDP5-s95V9DMENtVqJAVE3L-wO-NdDilyjyOmAbntgsCzYVGH9U3W_djh4t3qVFCv3r0S-DA2FD3THvlrFi655L0QHR3gu_Fbj3b9Ybtajpue_Q
	e: AQAB
	kid: 9Cquk0X-fNPSdePQIgQcQZtD6J0IjIRrFigW2PPK_-w
	n: utqtxbs-jnK0cPsV7aRkkZKA9t4S-WSZa3nCZtYIKDpgLnR_qcpeF0diJZvKOqXmj2cXaKFUE-8uHKAHo7BL7T-Rj2x3vGESh7SG1pE0thDGlXj4yNsg0qNvCXtk703L2H3i1UXwx6nq1uFxD2EcOE4a6qDYBI16Zl71TUZktJwmOejoHl16CPWqDLGo9GUSk_MmHOV20m4wXWkB4qbvpWVY8H6b2a0rB1B1YPOs5ZLYarSYZgjDEg6DMtZ4NgiwZ-4N1aaLwyO-GLwt9Vf-NBKwoxeRyD3zWE2FXRFBbhKGksMrCGnFDsNl5JTlPjaM3kYyImE941ggcuc495m-Fw
	p: 2zmGXIMCEHPphw778YjVTar1eycih6fFSJ4I4bl1iq167GqO0PjlOx6CZ1-OdBTVU7HfrYRiUK_BnGRdPDn-DQghwwkB79ZdHWL14wXnpB5y-boHz_LxvjsEqXtuQYcIkidOGaMG68XNT1nM4F9a8UKFr5hHYT5_UIQSwsxlRQ0
	q: 2jMFt2iFrdaYabdXuB4QMboVjPvbLA-IVb6_0hSG_-EueGBvgcBxdFGIZaG6kqHqlB7qMsSzdptU0vn6IgmCZnX-Hlt6c5X7JB_q91PZMLTO01pbZ2Bk58GloalCHnw_mjPh0YPviH5jGoWM5RHyl_HDDMI-UeLkzP7ImxGizrM

This is okay-ish for development purposes, but the project should eventually provide an example configuration file that states without ambiguity which configuration parameters are secrets that in a deploy process should be injected by a CI/CD pipeline, like this one.
https://github.com/italia/Satosa-Saml2Spid/blob/8272442b122ddf19d80ff8cfd00226f61d170d1a/example/plugins/backends/saml2_backend.yaml#L10-L11
This should be done so that uninformed developers do not accidentally go to production with an unsecure signing key

[peppelinux]

We should move the entire backend configuration using !ENV to facilitate configuration using docker compose and its env context