feat: add custom OAuth provider support

[junaiddshaukat]

Summary

Adds non-breaking support for custom OAuth providers configured in the InsForge dashboard, mirroring the approach already implemented in the JS SDK and Swift SDK.

This work stems from InsForge/InsForge#859 which added backend support for custom OAuth providers with configurable credentials and OIDC discovery endpoints.

Changes

• getOAuthUrl(providerKey: String, redirectUri, codeChallenge) — new overload that routes to /oauth/custom/{key} for dashboard-configured custom providers
• signInWithOAuthPage(providerKey: String, redirectUri) — new overload with full PKCE flow, generates code verifier/challenge and launches browser
• OAuthProvider enum is completely untouched — no source-breaking changes, existing consumers are unaffected
• Uses Ktor's appendPathSegments("oauth", "custom", providerKey) for URL construction — provider key is encoded exactly once as a single path segment (avoids double-encoding and slash pass-through issues)

Usage

// Built-in provider (unchanged)
client.auth.signInWithOAuthPage(OAuthProvider.GOOGLE, "myapp://callback")

// Custom provider configured in dashboard (new overload)
client.auth.signInWithOAuthPage("auth0-acme", "myapp://callback")

[junaiddshaukat]

Hey @jwfing —> this PR adds custom OAuth provider support to the Kotlin SDK, completing the rollout across all three SDKs (JS, Swift, Kotlin).

Same non-breaking approach as the Swift PR: OAuthProvider enum is untouched, and two new overloads (getOAuthUrl and signInWithOAuthPage) accept a providerKey: String that routes to /oauth/custom/{key}. URL encoding is handled via Ktor's appendPathSegments — single encoding, no double-encoding issues.

Ready for review whenever you get a chance!