feat: Add basic support for custom cache providers

[justblender]

As of right now, TypeORM only allows to use built-in cache providers, this pull request adds an option to specify a custom cache provider (that should implement QueryResultCache spec) in the ConnectionOptions object:

await createConnection({
  // ...all of the required options for establishing connection

  cache: {
    cacheProvider(connection) {
      return new MockQueryResultCache(connection);
    }
  }
});

[pleerock]

rename it to just "provider". We already have a "cache" word in a parent.

[pleerock]

Thanks for contribution! Can you please add some test that will check this functionality and docs to describe it for others

[justblender]

Smh, made a typo in the commit name.

@pleerock, I've added information in the docs and renamed the property, but I'm not used to write the tests. 😞

[pleerock]

Its never too late to learn :) Just copy paste exist test, replace few lines of code and it work.

[pleerock]

Thanks!