[Module Proposal]: `avm-res-managedservices-registrationdefinition`

### Check for previous/existing GitHub issues/module proposals

- [X] I have checked for previous/existing GitHub issues/module proposals.

### Check this module doesn't already exist in the module indexes

- [X] I have checked for that this module doesn't already exist in the module indexes.

### Bicep or Terraform?

Terraform

### Module Classification?

Resource Module

### Module Name

avm-res-managedservices-registrationdefinition

### Module Details

AVM Resource Module for Registration Definition (Lighthouse)

### Do you want to be the owner of this module?

No

### Module Owner's GitHub Username (handle)

_No response_

### (Optional) Secondary Module Owner's GitHub Username (handle)

_No response_

Start by reviewing the existing module indexes and prior module proposals for the expected Terraform AVM structure. The requested outcome is an AVM resource module for Registration Definition (Lighthouse), but this issue does not identify files, tests, implementation scope, or an owner.

Written by the indexing model from the issue text.