Persist mysqlrouter user across pod restart except for upgrades

[carlcsaposs-canonical]

Purpose: reduce number of DDLs

context: https://chat.canonical.com/canonical/pl/ba3ssouffirg8dfsob9mijae1o (discussion of canonical/mysql-k8s-operator#345)

[github-actions]

https://warthogs.atlassian.net/browse/DPE-3248

[taurus-forever]

@carlcsaposs-canonical please consider to address https://github.com/canonical/mysql-router-k8s-operator/issues/156 together with this issue. Tnx!

[carlcsaposs-canonical]

testing jira sync by closing & re-opening issue

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-8055.

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