jwt passphrase - provide a way to set passphrase without storing it in the context config on disk

The private key passphrase should not be stored in the config on disk, but the `aio-lib-ims-jwt` ims plugin requires it as a property on the context to unlock the private key. This is not secure. The passphrase is often a transient value provided as a flag via the CLI.

We need a way to:
1. use an in-memory-context config that can be modified without it being written to disk (so you can set the passphrase property, for example)
2. `getToken` should use this in-memory-context config