Allow SDKSynchronizer to user arbitrary checkpoints instead of bundled ones

[pacu]

Is your feature request related to a problem? Please describe.

This is something that would enable supporting Regtest checkpoints without the need of bundling them inside the SDK. See https://github.com/Electric-Coin-Company/zcash-android-wallet-sdk/issues/1216 and https://github.com/Electric-Coin-Company/zcash-swift-wallet-sdk/pull/1331. for reference

Describe the solution you'd like

Make checkpointTool extend from an interface that can be used to inject the custom checkpoint provisioning in testing environments