didcomm-jvm library doesn't have latest ACK header implementation

[ArtemPivovarov]

What

Current didcomm-jvm implementation treats please_ack header as a boolean and ack header as a string.
According to the current specs both headers should be treated as array of strings (IDs).

Why

Because of that, original ack and please_ack headers cannot be used in current dependent applications with respect to current specification

Success Criteria

ack and please_ack headers treated as array of strings.

Additional info

Implementation can be found in Message class

data class Message(
    ...
    val pleaseAck: Boolean?,
    val ack: String?,
    ...
)