feat(sdk): add per-request user impersonation to Context

Adds ctx.impersonate() with callback (auto-revert) and explicit
(caller-managed) modes. Borrows a dedicated pool connection via
Kysely's .connection() API, switches identity with dialect-specific
SQL (EXECUTE AS USER for MSSQL, SET ROLE for PostgreSQL), and
guarantees revert even on throw. Unsupported dialects throw at
call time. Includes username validation and dialect-specific quoting
for SQL injection prevention.

Author: damusix

.changeset/gentle-birds-impersonate.md

@@ -0,0 +1,9 @@
+---
+"@noormdev/sdk": minor
+---
+
+## Added
+
+* `feat(sdk):` Per-request user impersonation via `ctx.impersonate()` — borrow a dedicated pool connection, switch database identity, and run queries as a specific principal with guaranteed revert
+* `feat(sdk):` Callback mode (auto-reverts on completion or throw) and explicit mode (caller-managed lifecycle for cross-boundary use cases like Hapi request hooks)
+* `feat(sdk):` MSSQL (`EXECUTE AS USER` / `REVERT`) and PostgreSQL (`SET ROLE` / `RESET ROLE`) dialect support with SQL injection prevention via username validation and dialect-specific quoting