Allow loading keysets from files

[Volky]

Please add support for loading keysets from files, so it's easier to migrate from 1.x

[Florent Morselli]

If you are using Symfony 3.4+, it can be acheived using the new env var processors.

Ex. if your key set is stored in /path/to/signature.jwkset, then simply use %env(file:SIGNATURE_KEYSET)% in the bundle configuration and the env var SIGNATURE_KEYSET=/path/to/signature.jwkset.

I will document that in a migration guide.
If you are using Symfony 3.3, I do not see any way to solve that issue at the moment.

[Florent Morselli]

Closing as answered.
I will not include that feature for symfony 3.3 as this version is not maintained anymore.
The env var processor available from Symfony 3.4|4.x should be enough.