feat(snap): add support for environment variable injection

[farshidtz]

For details on the new scheme for setting environment variables, please refer to edgexfoundry/edgex-go#3986.

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/edgexfoundry/app-rfid-llrp-inventory/blob/main/.github/Contributing.md

PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?): Add tests for snap config options canonical/edgex-snap-testing#53
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

1. Set up:

snap install edgexfoundry --edge
snap install edgex-device-rfid-llrp --channel=edge/pr-75

2. Enable config and set a global config value:

$ snap set edgex-device-rfid-llrp config-enabled=true
$ snap set edgex-device-rfid-llrp config.service-startupmsg="testing injection"

$ snap start edgex-device-rfid-llrp
Started.

$ snap logs -n=all edgex-device-rfid-llrp | grep "testing injection"
2022-04-25T21:21:48+02:00 edgex-device-rfid-llrp.device-rfid-llrp[592246]: level=INFO ts=2022-04-25T19:21:48.571255952Z app=device-rfid-llrp source=variables.go:352 msg="Variables override of 'Service.StartupMsg' by environment variable: SERVICE_STARTUPMSG=testing injection"
2022-04-25T21:21:49+02:00 edgex-device-rfid-llrp.device-rfid-llrp[592246]: level=INFO ts=2022-04-25T19:21:49.102146213Z app=device-rfid-llrp source=message.go:55 msg="testing injection"

3. Set a app-specific value:

$ snap set edgex-device-rfid-llrp apps.device-rfid-llrp.config.service-port=11111

$ snap restart edgex-device-rfid-llrp
Restarted.

$ snap logs -n=all edgex-device-rfid-llrp | grep "11111"
2022-04-25T21:14:55+02:00 edgex-device-rfid-llrp.device-rfid-llrp[584932]: level=INFO ts=2022-04-25T19:14:55.14407642Z app=device-rfid-llrp source=variables.go:352 msg="Variables override of 'Service.Port' by environment variable: SERVICE_PORT=11111"
2022-04-25T21:14:55+02:00 edgex-device-rfid-llrp.device-rfid-llrp[584932]: level=INFO ts=2022-04-25T19:14:55.184841881Z app=device-rfid-llrp source=httpserver.go:123 msg="Web server starting (localhost:11111)"

New Dependency Instructions (If applicable)

[codecov-commenter]

Codecov Report

Merging #75 (d06d514) into main (5b207c8) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   41.08%   41.16%   +0.07%     
==========================================
  Files          21       21              
  Lines        9016     9016              
==========================================
+ Hits         3704     3711       +7     
+ Misses       4143     4138       -5     
+ Partials     1169     1167       -2     

Impacted Files	Coverage Δ
internal/llrp/reader.go	41.24% <0.00%> (+0.66%)	⬆️
internal/retry/retry.go	94.65% <0.00%> (+3.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b207c8...d06d514. Read the comment docs.

[farshidtz]

The commit is from @MonicaisHer. I have reviewed, tesed the changes, and created this PR.