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

[shazron]

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

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-844

[kwin]

I opened adobe/aio-lib-core-config#52 to support storing sensitive values in the system's keychain.