Add constant to process map

[antho1404]

Add support of constants for mapping.

Constants are helpful to get the user inputs, specific value, specific function for conversion using service-js, add value that is not part of any result...

It is only possible to set either a ref or a constant for a mapping.

To test:

Services

# serviceA
{"name":"serviceA","tasks":[{"inputs":[{"type":"String","key":"message"}],"outputs":[{"type":"String","key":"value"}],"key":"taskX"},{"key":"emits"}],"events":[{"data":[{"type":"String","key":"message"}],"key":"eventX"}],"configuration":{},"hash":"6QkdJaTHbx3qYT4H169twd7A2pzxDJt8MTy882h9nevn","sid":"serviceA","source":"QmTQW4Dnq44oU2DLq5dxXCHRNEQWaDaGc9ZKDmG1ocJGuv"}

# serviceB
{"name":"serviceB","tasks":[{"inputs":[{"type":"String","key":"inputA"}],"outputs":[{"type":"String","key":"res"}],"key":"taskY"}],"configuration":{},"hash":"j64r3wPevCd8f1vcZVNePBvnwRw6kZKdunAtRtd9hCY","sid":"serviceB","source":"QmQu8kCwPJ1Qydrawmz4STkY4FAwxrZHgkui83umkp94iq"}

Process

key: Event
steps:
  - type: trigger
    instanceHash: H74Qqq8nT5JZ9GSJmuSWLN5benWZPkUb5pYcvQLsoZX 
    eventKey: eventX
  - type: task
    instanceHash: DTjy36ApV5YVbyXm67iWeAsD9uCbZETcPL1TbUPwHeYJ
    taskKey: taskY
    inputs:
      inputA: hello world

You can use the CLI from this Pull Request mesg-foundation/cli#153 in order to test with the API updated

[krhubert]

ref will never be nil here (look above if check)

[antho1404]

good catch, got some issue with a conflict while rebasing may be coming from that.