Remove the default serializer in the matrix-sdk-store-encryption crate

The default serialization format for encrypted values in the store-encryption crate is JSON using serde_json. Since the ciphertext and nonce are byte vectors and JSON a textual encoding this ends up using more storage than what would be needed.

People can already use any other serialization format that is supported by Serde so let's just remove the default one.

Start in the matrix-sdk-store-encryption crate and inspect how serde_json is configured as the default serialization format for encrypted values. Confirm that other Serde-supported formats remain usable; done means JSON is no longer the default and the crate still handles its ciphertext and nonce values correctly.

Written by the indexing model from the issue text.