Add schema type for byte arrays in hex format

[Bargsteen]

Add a schema type for byte arrays in hex format. They should be represented with a JSON string.

This is especially useful with the new cryptographic primitives: https://github.com/Concordium/concordium-rust-smart-contracts/pull/101

Will also require matching changes in cargo-concordium and concordium-client.

This will introduce a new contract schema version and require updating

• Update in concordium-contracts-common
• Support in cargo-concordium
• Support in concordium-client
• Update contract schema documentation.
• Support in concordium-node-sdk-js

[limemloh]

This will be solved together with another addition to the schema. The current state of this can be tracked at https://github.com/Concordium/concordium-wasm-smart-contracts/issues/53