Add basic one-shot tera support, to enable use of environmental varia…

[peter-oneill]

Alternative to PR for regex-based implementation

Why this change?

This allows users to use Tera templating for additional flexibility with their floki.yaml. The target use case is to reference environmental variables in floki.yaml, for example to mount a $HOME subdirectory in a container as

docker-switches:
  - -v {{ get_env(name='HOME' }}/.vim:/home/build/.vim

Relevant testing

• New UT added,
• Tested manually against a local floki.yaml file which successfully mounted a directory declared as {{ get_env('HOME) }}, and similarly printed a host environmental variable in the container in an init command.

Contributor notes

See Teams chat for idea discussion.
Subsequently discussed using Tera with Max

Checks

• New/modified Rust code formatted with cargo fmt
• Documentation and README.md updated for this change, if necessary
  • Documentation, not readme, as this is more advanced use.
• CHANGELOG.md updated for this change

[maxdymond]

Fixed in another PR.