chore: allow fully overriding response body bytes in Modifier

[dmitryabramov-f3]

Currently pact-proxy is using https://github.com/tidwall/sjson to modify response body which allows overriding only specific keys in response body JSON, but not the whole body. The use case is to be able to dynamically modify some plain text responses, for example by specifying it as base64 encoded bytes.

This PR adds a handling for a special case path == "$.bytes.body" to override the whole body with the provided value. In this case bytes body will be passed base64 encoded so we have to base64 decode it.

Also add error handling when json.Marshal does not work on AddModifier in the client package.