Implement API support to /jobs endpoint

[rolfsimoes]

Links to #21, #18

Implement api_jobs() function to support /jobs endpoint.
Please, see OpenEO documentation on jobs endpoint.

[rolfsimoes]

Jobs must be run in new R process. We can start a new process using the package callr. The package provides basic functionality to interact with R process and can be used to build some dynamic fields of jobs endpoint response.

# Get all needed parameter to create a new API object
params <- api_params(api)

# params content:
# list(id = ..., title, description, backend_version, api_version, processes_file, collections)

# Create a new api object based on input parameters (to be run in **new process** side):
api <- api_from_params(params)

# This should be an openeocraft function
api_new_job <- function(api, req, res) {
  user <- user_session(req)
  p <- req$body
  if ("process" %in% names(p))
    p <- p$process
  job <- job_create(api, p, user)
  job_save(job) # user is a variable that identifies the current user
  return(job)
}

# Possible implementation of `job_start()`
job_start <- function(job) {
  callr::r(function() {
    api <- job_api(job)
    p <- job_pgraph(job)
    result <- run_pgraph(api, p)
    job_save_result(job, result) # user workspace
  })
}

The endpoint implementation (plumber.R) could be:

#* @post /jobs
function(req, res) {
  job <- api_new_job(api, req, res)
  job_start(job)
  return(job_info(job))
}

#* @get /jobs/<job_id>
function(req, res, job_id) {
  job <- job_read(api, res, req, job_id)
  return(job_info(job))
}

#* @delete /jobs/<job_id>
function(req, res, job_id) {
  job <- job_delete(api, res, req, job_id) # inside job_delete: user <- user_session(req)
  return(job_info(job))
}

Possible new functions:

• job_save_result()
• job_api()
• job_save()
• job_read()
• job_delete()
• job_create()
• job_api_params()
• job_start()
• job_info()

This issue depends on #18 to be able to define workspace.

[rolfsimoes]

[PondiB]

Possible new functions:

• job_save_result()
• job_api()
• job_save()
• job_read()
• job_delete()
• job_create()
• job_api_params()
• job_start()
• job_info()

• job_update()
• job_get_results()
• jobs_list_all()