Make the example config file into a template and move it to arti

This is the doable parts (points 1 and 2) of #457 (closed).

assigned to @Diziet

added 15 commits

• 37fcd5be...5990df53 - 6 commits from branch tpo/core:main
• 58f53bae - config defaults: Test that going via builder explicitly is the same
• afa64bf3 - config defaults: No longer apply ARTI_DEFAULTS in load()
• c9c8344b - ARTI_DEFAULTS: Move to arti crate
• 8aab5a39 - ARTI_EXAMPLE_CONFIG: Rename from ARTI_DEFAULTS
• 1105e631 - config defaults: Test that empty deser is the same
• 85a310d7 - arti-example-config: Mark some non-default examples more clearly
• f18373a7 - arti-example-config.toml: Comment out all the example settings
• bb166c14 - arti-example-config.toml: Re-un-comment the example settings in test
• cb42c01d - arti-example-config.toml: Add a slightly improved comment

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Rebased to sort out conflict with my own MR.

requested review from @nickm

added 1 commit

• d407ce51 - Add semver note for these changes

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Looks fine. The only comments I had were about changes I wanted to the tests... and then you went and made those changes in subsequent commits.

merged

mentioned in commit 1698a2fe