Add extension to inject environment variables into snaps by it's command-chain.
This extension takes snap options and transform then into environment variables.
For the env-exporter program, Rust Tier 2 toolchains are marked as unsupported.

Signed-off-by: Lincoln Wallace <lincoln.wallace@canonical.com>

* test: add spread tests for env-injector extension

- add: spread test:
  * hello.c: application which consumes env variables
  * task.yaml: Tests to verify env-injector functionality

Signed-off-by: Lincoln Wallace <lincoln.wallace@canonical.com>

Signed-off-by: Callahan Kovacs <callahan.kovacs@canonical.com>

- doc: Add line break on Pydoc
- rust exporter: Fix error messages
- spread: Add case to test app-specific envfile
- spread: Fix test for verify alias overrite behavior
- fix: typo error
- fix: commentaries not being discarted in envfiles
- feat: add logs to check test execution
- fix: shellchek error
- fix: use absolute path for envfiles
- fix: address lint errors
- fix: PEP8 formating
- exporter program moved to:
  https://github.com/canonical/snappy-env

Signed-off-by: Lincoln Wallace <lincoln.wallace@canonical.com>
Co-authored-by: Farshid Tavakolizadeh <email@farshid.ws>
Co-authored-by: Callahan <callahan.kovacs@canonical.com>

Signed-off-by: Lincoln Wallace <lincoln.wallace@canonical.com>