Adapt Keystore logic to be compatible with fips mode

## Problem Description
In https://github.com/elastic/beats/pull/42846 the keystore was removed from beats for ensuring functionality problematic for fips is not pulled into the beats binaries. This was a stop gap solution, but ideally the keystore implementation would be adopted to be usable in fips mode.

## Goal
Review the keystore implementation and adapt implementation to be compatible with fips mode.
Initial stab in https://github.com/elastic/elastic-agent-libs/pull/277