chore: Remove Gson dependency

[felipecsl]

Description

This is a breaking change, as it changes the public signature of EppoClient#getJSONAssignment, so we should bump the major version to v4.0.0 to reflect that.

This depends on Eppo-exp/sdk-common-jdk#7

Summary of changes

• Removed dependency on Gson
• Replaced parsing with Jackson instead, which was also already a dependency
• Fixed tests
• Made some classes immutable

[aarsilv]

Digging the immutable DTOs! This is awesome same minor questions as the other: should we be using org.json so Android builds don't need an extra package?

[felipecsl]

Digging the immutable DTOs! This is awesome same minor questions as the other: should we be using org.json so Android builds don't need an extra package?

Synced offline, TL;DR yes we're exposing org.json.JSONObject here https://github.com/Eppo-exp/android-sdk/blob/main/eppo/src/main/java/cloud/eppo/android/EppoClient.java#L353-L355