Create pre-flight validation module

Add comprehensive pre-flight checks to validate environment before
replication starts:

- Local environment: pg_dump, pg_dumpall, pg_restore, psql availability
- Version compatibility: local pg_dump version vs source server
- Network connectivity: test connections to source and target
- Source permissions: REPLICATION privilege and table SELECT access
- Target permissions: CREATEDB and subscription privileges

Results are collected into PreflightResult with formatted output
showing pass/fail status and suggested fixes for issues.

Closes #5

Author: taariq

src/lib.rs

@@ -11,6 +11,7 @@ pub mod migration;
 pub mod mongodb;
 pub mod mysql;
 pub mod postgres;
+pub mod preflight;
 pub mod remote;
 pub mod replication;
 pub mod sqlite;