feat(snap): Support for snap options to set environment variables

[farshidtz]

This PR will add support for setting environment variables for the service, via snap options.

In addition:

• The install hook has been ported from bash to go (since the added configure hook is also in Go)
• The install mode is changed to disable which means the service no longer starts by default. This is needed to allow setting the environment variables before the service start, dropping the need for a manual restart. This change may be considered a breaking change and needs further investigation. In particular, need to test if an upgrade from an older version results in a stopped service or not.

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/edgexfoundry/edgex-ui-go/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: Remove UI service online requirement in setup canonical/edgex-snap-testing#121
• 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: feat: Extend EdgeX UI snap getting started for disabled service and security edgex-docs#868

Testing Instructions

snap install edgex-ui --channel=latest/edge/pr-553
snap set edgex-ui config.edgex-security-secret-store=false
snap start edgex-ui

snap install edgexfoundry
snap set security-secret-store=off

Access http://localhost:4000

New Dependency Instructions (If applicable)

The new dependency (github.com/canonical/edgex-snap-hooks/v2) has been approved in the past and used in other Go projects:

[MonicaisHer]

Verified backward compatibility:

$ snap install edgex-ui
$ snap refresh edgex-ui --channel=latest/edge/pr-553
$ snap services edgex-ui
Service            Startup  Current  Notes
edgex-ui.edgex-ui  enabled  active   -

$ snap set edgex-ui config.edgex-security-secret-store=false
$ snap restart edgex-ui
$ snap install edgexfoundry

Access http://localhost:4000/ successfully

[MonicaisHer]

@farshidtz Thank you, looks good.