fix: #879

• Use the current service writable config as the baseline for the initial update
• Store previousCommonWritable as map to avoid pointer reference syncing with Config Provider

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?)
• 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. Build device-virtual using this version of go-mod-bootstrap
2. Deploy core services and device-virtual
3. Use Core Keeper API to update a common writable configuration key. For example:

curl -X PUT http://localhost:59890/api/v3/kvs/key/edgex/v4/core-common-config-bootstrapper/all-services/Writable/Telemetry/Interval \
  -H "Content-Type: application/json" \
  -d '{"value": "20s"}'

4. The first config update should succeed and output this log:

level=INFO ts=2025-07-04T16:02:11.242333+08:00 app=device-virtual source=config.go:1029 msg="Writable configuration has been updated from the Configuration Provider"
level=INFO ts=2025-07-04T16:02:11.24244+08:00 app=device-virtual source=config.go:1052 msg="Telemetry interval has been updated. Processing new value..."
level=INFO ts=2025-07-04T16:02:11.242467+08:00 app=device-virtual source=manager.go:47 msg="Metrics Manager report interval changed to 20s"

rather than the previous:

level=WARN ts=2025-07-02T09:36:30.330645+08:00 app=device-virtual source=config.go:931 msg="ignoring changed writable key InsecureSecrets overwritten in App or Device common writable"