hcs-relay: uses duplicated code for mirror subscriptions

**Detailed Description**
The HCS relay plugin uses its own (duplicated) code for mirror subscriptions

**Actual Behavior**
Code in HCS relay is a duplicate of the mirror-direct plugin.

**Expected Behavior**
HCS relay uses the plug in, rather than duplicating code.
Should use the same run time plug-in selection to allow for alternative mirror node apis to be supported by relay.

**Environment:**
- Java: [e.g. OpenJDK 11.0.4]
- Node: [e.g. v12.9.1-x86]
- OS: [e.g. Ubuntu 18.04]
- Version: [e.g. 1.0.1]

**Additional Context**
Add any other context about the problem here. Attach any logs here, if applicable.