feat: implement closeAccount method

[beeman]

This PR implements a way to close accounts, it builds on top of the feature added in #502.

The actual closeAccount method consists mainly of a bunch of validations that would prevent the account from being closed.

Closes #308.

Follow-up:

Implement in the other SDKs:

• [ ] Android https://github.com/kin-labs/kinetic-android-sdk/issues/18
• [ ] Dart https://github.com/kin-labs/kinetic-dart-sdk/issues/15
• [ ] iOS https://github.com/kin-labs/kinetic-ios-sdk/issues/7
• [ ] Python https://github.com/kin-labs/kinetic-python-sdk/issues/31
• [ ] Unity https://github.com/kin-labs/kinetic-unity-sdk/issues/13

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