Backend: Convert input params into hash

Summary

Part of: Add support for spec:inputs in GitLab CI pipelines (&16321 - closed)

Update params into a hash when reading from GraphQL

pipeline_inputs = {}

input_params = ... # read from graphql mutation arguments
input_params.each do |input| 
  pipeline_inputs[input.name] = JSON.parse(input.value)
end

Ci::CreatePipelineService.new(...).execute(..., inputs: pipeline_inputs)