Add JKS (Java KeyStore) support?

[rmrustem]

Hello,

I would like to contribute adding JKS (Java KeyStore) support to the exporter, if you're interested. That would allow to get expiration metrics from certificates stored in JKS containers, which are typically used in Java applications, including infrastructure software such as Kafka, Elasticsearch, etc.

That can be done as a separate module similar to file or right inside the file module. Thoughts?

[rmrustem]

Oh, there's already #141