Add support for the Jamf Platform API gateway

[Jordy Thery (JordyThery)]

Important

Depends on #137 (App Installers support). That PR's commit is included in this branch as its base — please review and merge #137 first; this PR will then contain only the single "Add support for the Jamf Platform API gateway" commit (rebase on request).

Summary

Adds the ability to connect to servers through the Jamf Platform API gateway using an integration created in Jamf Account, as an alternative to connecting to a Jamf Pro instance directly.

No new UI: the gateway region URL with the platform environment id appended is entered as the server address, together with the existing Use API Client option:

URL:           https://us.api.jamfcloud.com/<environmentId>
Use API Client: checked
Client ID:      <integration client id>
Client Secret:  <integration client secret>

The gateway can be used for the source and destination independently (including mixed gateway/direct migrations), from the UI and the command line (help text updated).

How it works

• ApiRequest (new) centralizes endpoint URL and request header construction, previously repeated across the files making API calls. Call sites keep their exact URLs, auth styles, and content types — no functional change for direct connections.
• When the server is a gateway (*.api.jamfcloud.com), ApiRequest routes Classic API calls through /proclassic/... and Jamf Pro API calls through /pro/..., and sends the X-Environment-Id header with every request (including multipart icon uploads).
• Tokens are requested from the gateway token endpoint (/auth/token, client credentials, 900 second lifetime); the existing renewal logic handles the shorter lifetime unchanged.
• The environment id is parsed from the server address in baseUrl() and kept per server, so stored credentials, the server list, and the last-user record all key off the gateway + environment combination.

Testing

Against a Jamf Pro 11.32 via both EU and US gateway (platform environment scope integration):

• ✅ Bulk, selective, and delete (removal) with the gateway configured on both source and destination
• ✅ Mixed mode: gateway source → direct destination and vice versa
• ✅ Export to files and command line (-source https://us.api.jamfcloud.com/<environmentId> -sourceUseClientId true ...)
• ✅ Direct (non-gateway) connections regression-tested — URLs and headers are byte-identical to before
• API-level validation of the gateway: Classic XML GET/POST/PUT/DELETE, paged Jamf Pro API, icon upload (/pro/v1/icon), JCDS listing

Notes / limitations

• The gateway has no sticky sessions; rapid read-after-write can briefly 404 (existing retry logic applies). Noted for documentation rather than code.
• Legacy on-prem iconservlet icon URLs are not reachable through the gateway; Jamf Cloud instances serve icons from the public icon CDN, which works unchanged.

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