Replace `notion-sdk-jvm` with custom Spring Rest Client

[scordio]

Currently, Notion SDK JVM is used as an underlying dependency to call the Notion API.

Although it gives the benefit of accessing the API from Java without additional effort, it has a few downsides:

• Brings Gson and Kotlin transitively
• Does not have a module descriptor
• Could slow down the adoption of new API types released by Notion

An alternative could be to adopt one of the Rest Client options that Spring provides.

[scordio]

Hi @fahmikudo, regarding scordio/spring-batch-notion#37 (comment):

Interested
Can you explain which part needs to be improved integration test? Or maybe there is an explanatory document?
Maybe if I have free time I will try to contribute.

In short, the notion-sdk-jvm module I'm using hides away the complexity of calling the Notion REST API.

I introduced some tests based on Wiremock, but they don't provide comprehensive coverage for what notion-sdk-jvm does under the hood.

For this reason, before taking up this issue, I would like to introduce more robust tests that could ideally point to an actual database instance on notion.so. I haven't had the time to sketch such tests, but I'll let you know once I come up with something.