allow yaml internals to specify custom inputs

[rfbgo]

This adds support for declaring custom workloads and custom inputs at the yaml level, eg:

  applications:
    hostname:
      workloads:
        custom_analysis:
          experiments:
            sweep_dataset_{dataset_num}:
              matrix:
                - dataset_num
              variables:
                dataset_num: ['1', '2']
              internals:
                custom_inputs:
                  sim_pkg:
                    url: 'file://{workspace_root}/sample_inputs/sim_package.tar.gz'
                    sha256: null
                    expand: true
                  dataset_file:
                    url: 'file://{workspace_root}/sample_inputs/dataset_{dataset_num}.txt'
                    sha256: null
                    expand: false
                custom_executables:
                  show_dataset:
                    template:
                      - 'echo "=== Input Dataset ({dataset_file}) ==="'
                      - 'cat {dataset_file}'
                    use_mpi: false
                  show_pkg_config:
                    template:
                      - 'echo "=== Extracted Package Config ({sim_pkg}/config.cfg) ==="'
                      - 'cat {sim_pkg}/config.cfg'
                    use_mpi: false
                custom_workloads:
                  custom_analysis:
                    executables:
                      - local
                      - show_dataset
                      - show_pkg_config
                    inputs:
                      - sim_pkg
                      - dataset_file
                    tags:
                      - custom_demo
        local:
          experiments:
            override_builtin_demo:
              internals:
                custom_executables:
                  local:
                    template:
                      - 'echo "=== Overridden Built-in Local Workload on $(hostname) ==="'
                    force: true

[ramble-project-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: 4ad6f98

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.0361	2.0776 (2d42cdb)	2.0633
test_large_template_expansion	passed	1.1699	1.1994 (2d42cdb)	1.1989
test_many_experiments	passed	23.0408	23.2386 (2d42cdb)	23.6468
test_many_objects_defaults	passed	12.0834	12.1959 (2d42cdb)	12.5012
test_matrix_filter_perf	passed	1.0020	1.0590 (2d42cdb)	1.0757