feat: add Client Management and Network Information, completing both API areas

[mikemadeja]

Summary

• Adds Get-PiHoleClient, New-PiHoleClient, Update-PiHoleClient, Remove-PiHoleClient, and Get-PiHoleClientSuggestion, covering Pi-hole's client management API. Single-item DELETE is intentionally skipped in favor of the batch endpoint, same precedent as Remove-PiHoleList/Remove-PiHoleDomain.
• Adds Get-PiHoleNetworkGateway, Get-PiHoleNetworkRoute, Get-PiHoleNetworkInterface, Get-PiHoleNetworkDevice, and Remove-PiHoleNetworkDevice, covering the full Network Information API. Unlike Clients/Domains/Lists, there's no batch-delete alternative for devices, so Remove-PiHoleNetworkDevice implements the single-item DELETE directly.
• Interface/route/gateway responses vary significantly by type and aren't fully enumerable from the OpenAPI spec, so they're returned via the module's existing generic PascalCase-conversion helper (already used by Get-PiHoleConfig) rather than a hand-picked set of fields. Confirmed against the real server this also correctly surfaces at least one live field not documented in the spec (interface.stats.bits).
• Adds "Client Management" as its own README/docs/EXAMPLES.md category; Network Information folds into "Configuration & Diagnostics" like the module's other read-only diagnostic areas (FTL Information, PADD, Teleporter, History).

This completes both the "Clients" and "Network" areas from the API coverage audit (12 of 12 operations addressed: 11 implemented, 1 intentionally skipped).

Test plan

• All behavior verified directly against the real Pi-hole server before writing tests (create, duplicate detection, get/filter, group assignment including invalid-group-name rejection, update-merge, not-found errors, batch/single remove, confirm-gone).
• 41 new Pester integration tests across the 10 functions (23 Clients + 18 Network), all passing against the real server.
• Confirmed server left in a clean state after every test run and after regenerating docs/EXAMPLES.md.
• Invoke-ScriptAnalyzer clean against the full module.
• Full non-integration Pester unit suite passes (4/4; unaffected by this change).

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