Add snapshot support to postgres module

[Nikola-Milovic]

Added snapshot and restore feature to Postgres module, very closely following the Golang implementation

Closes #952

Summary:

• ✔️ Snapshot and restore functionality
• ✔️ Tests
• ✔️ Docs updated and previewed (copied over from golang)
• ✔️ Lint & format
• ❌ Haven't updated the package version

What can be improved:
I utilized comments very heavily, but it really helps with the glance value of the tests, you can very easily identify what is happening, but I can remove them if requested.

Tests:

 ✓ src/postgresql-container.test.ts (10 tests) 27393ms
   ✓ PostgreSqlContainer > should connect and return a query result  2957ms
   ✓ PostgreSqlContainer > should work with database URI  2466ms
   ✓ PostgreSqlContainer > should set database  3059ms
   ✓ PostgreSqlContainer > should set username  2399ms
   ✓ PostgreSqlContainer > should work with restarted container  3013ms
   ✓ PostgreSqlContainer > should allow custom healthcheck  762ms
   ✓ PostgreSqlContainer snapshot and restore > should create and restore from snapshot  3334ms
   ✓ PostgreSqlContainer snapshot and restore > should use custom snapshot name  3314ms
   ✓ PostgreSqlContainer snapshot and restore > should handle multiple snapshots  4100ms
   ✓ PostgreSqlContainer snapshot and restore > should throw an error when trying to snapshot postgres system database  1987ms

 Test Files  1 passed (1)
      Tests  10 passed (10)
   Start at  07:26:23
   Duration  27.81s (transform 167ms, setup 0ms, collect 310ms, tests 27.39s, environment 0ms, prepare 31ms)

 PASS  Waiting for file changes...
       press h to show help, press q to quit

cc @cristianrgreco

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[cristianrgreco]

Thanks @Nikola-Milovic! Just a few nitpicks but overall LGTM

[Nikola-Milovic]

Also made the js docs more concise