feat: enhance SMDClient with group membership caching and reverse indexing

[alexlovelltroy]

Description

This pull request enhances the SMDClient implementation by improving concurrency safety, adding efficient reverse lookup indexes for node identification, and supporting group membership information for nodes. It also updates the tests to match these changes. The main improvements are grouped below:

Concurrency and Data Structure Improvements

• Changed nodesMutex from a sync.Mutex to a sync.RWMutex throughout the codebase to allow multiple concurrent readers, improving performance for read-heavy workloads. [1] [2] [3] [4] [5] [6] [7] [8] [9]
• Added reverse lookup maps (ipToXname, macToXname, wgipToXname) to SMDClient for O(1) lookups by IP, MAC, or WGIP, and updated all relevant methods to use these indexes for faster node identification. [1] [2] [3] [4]

Node and Group Membership Enhancements

• Updated the NodeMapping struct to include a Groups field, and modified PopulateNodes to fetch and store group membership information for each node. The GroupMembership method now returns cached group data instead of querying the server each time. [1] [2] [3] [4]

Test Updates

• Updated tests to initialize the new reverse lookup maps and use sync.RWMutex. Enhanced test HTTP handlers to simulate group membership endpoints, ensuring group membership and reverse lookup logic are tested. [1] [2] [3] [4] [5] [6] [7]

These changes collectively improve the efficiency, scalability, and feature set of the SMDClient.

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
• I have run make test (or equivalent) locally and all tests pass
• DCO Sign-off: All commits are signed off (git commit -s) with my real name and email
• REUSE Compliance:
  • Each new/modified source file has SPDX copyright and license headers
  • Any non-commentable files include a <filename>.license sidecar
  • All referenced licenses are present in the LICENSES/ directory

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

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[travisbcotton]

Lgtm