WIP: feat/support decryption key alias

[joseph-neeraj]

PR checklist

• [ x ] An issue/feature request has been created for this PR
• [ x ] Pull Request title clearly describes the work in the pull request and the Pull Request description provides details about how to validate the work. Missing information here may result in a delayed response.
• [ x ] File the PR against the main branch
• [ x ] The code in this PR is covered by unit tests

Link to issue/feature request: add the link here

Description

• The README was missing the detail regarding using PKCS12 file directly in the config, instead of the private key. This has been fixed.
• The lib's json configuration was inconsistent with that of nodejs, nodejs uses keyStore and keyStorePassword for specifying the decryption key, and this lib uses decryptionKey and decryptionKeyPassword. Made these aliases so it's consistent now