[MNG-5689] Checksum policy for mirrors

**[Christopher Tubbs](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=ctubbsii)** opened **[MNG-5689](https://issues.apache.org/jira/browse/MNG-5689?redirect=false)** and commented

It does not appear that there is any way to configure a checksum policy for mirrors in the settings.xml file.

In particular, I'd love to enforce a "strict" checksum policy on maven central. I can configure a mirrorOf central, but I cannot set the checksum policy. This seems like a big oversight.

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**Affects:** 3.2.3

**Issue Links:**
- [MNG-5506](https://issues.apache.org/jira/browse/MNG-5506) Master override of checksumPolicy

- [MNG-4645](https://issues.apache.org/jira/browse/MNG-4645) Move Central repo definition out of Maven's jars so it can be more easily changed

- [MNG-5728](https://issues.apache.org/jira/browse/MNG-5728) Switch the default checksum policy from "warn" to "fail"

**Backported to:** [Issues to be reviewed for 4.x](https://github.com/apache/maven/milestone/117?closed=1)