feat(agent): add PSU Event Hub compatibility

[Marc-André Moreau (mamoreau-devolutions)]

Adds PowerShell Universal Event Hub remote agent compatibility as an optional Devolutions Agent feature. This lets Devolutions Agent connect to PSU Event Hubs, receive command/script invocations, and return PSU-compatible PowerShell results without installing a separate PSU agent service.

The feature also supports migration-oriented behavior for existing PSU agent configuration, including legacy config files, PSU_ environment overrides, $secret: AppToken references, and bounded PowerShell worker concurrency.

Draft until live PSU server compatibility testing and installer artifact validation are complete.

Issue: N/A

[Marc-André Moreau (mamoreau-devolutions)]

Implementation notes:

• Adds a Rust PSU Event Hub compatibility module under devolutions-agent/src/psu_event_hub with a minimal SignalR JSON client for PSU Event Hub invocations.
• Uses PowerShell itself for CLIXML compatibility through [System.Management.Automation.PSSerializer] instead of reimplementing PSU serialization in Rust.
• Supports {Hub}, {Hub}Void, {Hub}Module, {Hub}ModuleVoid, and GetResult semantics, including remove-on-read result storage.
• Loads native PsuEventHub config and imports legacy PSU config from %ProgramData%\PowerShellUniversal\eventHubClient.json, %ProgramData%\PowerShellUniversal\agent.json, and %AppData%\PowerShellUniversal\agent.json.
• Adds practical PSU_ environment compatibility and $secret: AppToken resolution via PowerShell Get-Secret.
• Installer changes expose the feature as an optional Devolutions Agent component and preserve existing feature JSON when toggling Enabled.
• Worker execution is bounded by MaxWorkerPoolSize; persistent worker pooling remains a future performance optimization if needed.

Validation performed:

• cargo +nightly fmt --all
• cargo clippy -p devolutions-agent --tests --offline -- -D warnings
• cargo test -p devolutions-agent --offline

Known gaps before marking ready:

• Live PSU server smoke/compatibility testing is still needed.
• Windows UseDefaultCredentials/Negotiate auth is still not implemented.
• Full local MSI packaging was started but not completed in this session; CI workflow was dispatched earlier for installer artifacts.

[github-actions]

Let maintainers know that an action is required on their side

• Add the label release-required Please cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module) when you request a maintainer to cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module)

• Add the label release-blocker Follow-up is required before cutting a new release if a follow-up is required before cutting a new release

• Add the label publish-required Please publish libraries (`Devolutions.Gateway.Utils`, OpenAPI clients, etc) when you request a maintainer to publish libraries (Devolutions.Gateway.Utils, OpenAPI clients, etc.)

• Add the label publish-blocker Follow-up is required before publishing libraries if a follow-up is required before publishing libraries