Provide tools for dealing with JSON parameters.

Key abilities gained in this PR:

• Read JSON as either string or file
  • Exciting potential when used with commandArgs(trailingOnly = TRUE), provides for reading the params directly, and can specify the Rscript as the ENTRYPOINT, and then the JSON as the argument in docker run
• Inherit other JSON files by specifying the directory where the file is as an argument to parse_params, and then specifying the basename in the inherit` key of the JSON (see tests for examples)
• Validate the parsed and inherited parameters against a JSON schema

An example of a "normal" call would be something along the lines of:

cfg <- parse_params(
  json = commandArgs(trailingOnly = TRUE),
  inheritence_search_paths = system.file("params", package = "workflow.pacta"),
  schema_file = system.file("schema", "params_schema.json", package = "workflow.pacta")
)