[Serializer] Add xml_format_output context option. Close #12517.
feature #13031 [Serializer] Add xml_format_output context option. Close #12517. (dunglas)

This PR was merged into the 2.7 branch.

Discussion
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[Serializer] Add xml_format_output context option. Close #12517.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12517
| License       | MIT
| Doc PR        | n/a

Allows to pretty print XML encoder output.

Commits
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37d3f84 [Serializer] Add xml_format_output context option. Close #12517.