feat: expose 'nr-path:agent_dir' when expanding values

[sigilioso]

This exposes the new variable ${nr-path:agent_dir} when expanding user values. This allows referencing additional configuration files from a particular configuration in agent types.

Example from the unit test:

agent-type:

name: first
namespace: newrelic
version: 0.1.0
platform: host
operating_system: linux
variables:
  config:
    description: ""
    type: string
    required: true
  extra_configs:
    description: ""
    type: string_map
    required: true
deployment:
  filesystem:
    extra_configs:
      kind: dir_content_from_map
      source: ${nr-var:extra_configs}
    config.toml:
      kind: file
      text: ${nr-var:config}
  executables:
    - id: first
      path: /opt/first
      args:
        - --config
        - ${nr-sub:filesystem_agent_dir}/config.toml

values:

config: |
  extra = ${nr-path:agent_dir}/extra_configs/extra.txt

extra_configs:
  extra.txt: |
    SOME CONTENT
#,

[gsanchezgavier]

thanks! i'm not sure how is the current docs about this , but it would be worth to check and document it

[gsanchezgavier]

nit: we could improve this comment with why we are expanding secrets and this var.