Add PostHog Cloud OAuth and self-hosted support

PostHog Cloud authenticates via CIMD OAuth (public client, PKCE)
through the region-agnostic oauth.posthog.com gateway, with an API-key
fallback. PostHog Self-Hosted is a separate provider using an API key
and an instance URL.

The shared driver discovers the data region by probing us/eu for OAuth
connections, since the gateway does not serve the data API, and pins
pagination to the resolved host.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/coredata/migrations/20260529T617403Z.sql

@@ -0,0 +1,15 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TYPE connector_provider ADD VALUE IF NOT EXISTS 'POSTHOG_SELF_HOSTED';