chore(publish) abstract environment variables and introduce feature flags

[dduportal]

While working on #777 for the Update Center migration to a mirror system, we realized the following problems with the current publish.sh script:

• It's hard to test a change until it's in production => we would want to enable/disable some steps to only focus testing on others in order to have enough confidence to merge PRs without breaking the production (neither requiring a fully fledged staging environment)
• There are parameters written in the script itself which requires SREs to open an (untestable for today) PR and merge it as soon as possible when rotating expired credentials/changing configurations of bucket, etc.

This PR is a proposal to solve these issues with:

• Feature flagging for the top-levels stages of this script, through en environment variable which has a default value (same behavior as today) and can be overridden by the caller
• Definition of the Update Center sync. tasks through an environment variable (same as above: default value which can be overridden) and a convention that each "sync" task will load their own environment file to get their parameters and credentials

Example of the expected source file structures to be set in a ZIP credential file:

####### File env-rsync-pkg
export UPDATES_SITE=updates.jenkins.io
export RSYNC_USER=mirrorbrain
export RSYNC_GROUP=www-data
export RSYNC_REMOTE_DIR=/var/www/updates.jenkins.io
export FILESHARE_SYNC_SOURCE=./www2/

####### File env-azsync-content
export STORAGE_NAME=updatesjenkinsio
export STORAGE_FILESHARE=updates-jenkins-io
export STORAGE_DURATION_IN_MINUTE=5
export STORAGE_PERMISSIONS=dlrw
export JENKINS_INFRA_FILESHARE_CLIENT_ID='<redacted>'
export JENKINS_INFRA_FILESHARE_CLIENT_SECRET='<redacted>'
export JENKINS_INFRA_FILESHARE_TENANT_ID='<redacted>'
export FILESHARE_SYNC_SOURCE=./www-content/

####### File env-s3sync-westeurope
export AWS_ACCESS_KEY_ID='<redacted>'
export AWS_SECRET_ACCESS_KEY='<redacted>'
export AWS_DEFAULT_REGION='auto'
export BUCKET_NAME=westeurope-updates-jenkins-io
export BUCKET_ENDPOINT_URL=https://8d1838a43923148c5cee18ccc356a594.r2.cloudflarestorage.com
export FILESHARE_SYNC_SOURCE=./www-content/

Note: This PR is ready to review but not to merge as the env. files need to be generated and set up AND no test done (yet) on real life

[dduportal]

We're merging this PR into a specific branch to allow testing. We'll open a subsequent PR for the final review to master branch. Any code review should happen in #787