Move index management concerns into the saved object client

[chrisdavies]

This is one in a series of several PRs that will go towards resolving #15100.

This introduces the following changes:

• Allows additive changes to existing mappings
• Moves getKibanaIndexMappingsDsl and general mapping-building logic into the saved object client system
• Initializes the Kibana index (patching its mappings and setting the index template) when the saved object client initializes
• Removes the index patching logic from the elasticsearch health check
• Removes the constant setting of the index template prior to saved object client writes

Testing this PR

• Kibana should fail to start, and give a clear error
  • If starting 6.x with a 5.x index
  • If multiple plugins define the same mapping
  • If a mapping type starts with a leading underscore (e.g. _foo)
• You should now be allowed to add properties to an existing mapping

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[kobelb]

We're no longer passing the onBeforeWrite to the ScopedSavedObjectsClientProvider constructor so when we set a custom client factory, we'll no longer have access to the onBeforeWrite.

Perhaps a sample usage makes this easier to follow: https://github.com/elastic/kibana/blob/security-app-privs/x-pack/plugins/security/index.js#L120

[kobelb]

I'm bad with words, so I threw up chrisdavies#1 to illustrate the changes that would make this possible.

[chrisdavies]

@kobelb Thanks! Merged your PR and master and made some adjustments. Have a look to see if it passes muster.

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[rhoboat]

Just dropping a note to say I'm backlogged on reviews, haven't forgotten, hope to get to it this week.

[chrisdavies]

@archanid I'm canning this one, as we've decided to write migrations in such a way that breaking changes are allowable. So, that changes this PR a bit.