Add inherit_metadata setting for translated pages

[bemyak]

This PR implements #3204

There were two ways of doing this:

1. Implement merging for already parsed pages. This would require making fields of PageFrontMatter into Optionals, which users can observe in serialized template. Also, the merge function must be updated every time a new field is added to PageFrontMatter. Empty arrays become indistinguishable from absent (e.g. for authors = []). Meh.
2. Merge the front matter after TOML parsing, but before being parsed as a page. This is much less impactful way, which I chose here. The drawback is that pages now need to carry the raw frontmatter.

The performance impact for an average size frontmatter (~10 fields) seems to be 45ms for 500 pages, so <1ms per page, part of which is newly rendered taxonomy pages. This is paid only when the feature is enabled, no change by default.

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Code changes

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• Are you doing the PR on the next branch?
• Was a LLM used? If so, for what? Yes, for writing tests, performing benchmarks and pondering ideas about feature integration, help with iterating over the code. I understand the submitted changes.

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• Have you created/updated the relevant documentation page(s)?