Describe the proposed change(s).

This requirement is primarily for use in the Mend app, but may also be useful to self-hosted scenarios too.

In the app we want to support three main concepts/modes:

• Silent: same as dryRun=lookup but also allows checkedBranches to trigger the creation of branches/PRs. Any pre-existing branches should remain open, be updated, etc. Another way of thinking of this is like prCreation=approval but without the dependency dashboard issue. It could alternatively be named like "Scan only" or "On demand"
• Remediation: same as silent (allows checkedBranches) but defaults to prCreation=immediate for remediation PRs, i.e. vulnerability fixes. This would be a first-class way to support the people who want "only security fixes" without needing to play with packageRules
• Full (default): same as today

The concept of dryRun would still exist and be compatible with and override the above. e.g. if -mode=silent and checkedBranches=renovate/foo and dryRun=lookup then the run wouldn't create any branch.

This mode should be user-configurable, e.g. if the user configures mode=remediation in renovate.json then it would override anything set by the app in global config.