Pull the flattened field type into a mapper plugin.

[jtibshirani]

This PR shows a prototype approach to pulling FlatObjectFieldMapper into its
own MapperPlugin. It introduces two new methods on FieldMapper:

• supportsKeyedFieldLookup(), which specifies whether the field type supports
  looking for subfields
• keyedFieldType(String key), which gives the opportunity to return a special
  field type for a subfield

FieldTypeLookup has been updated to refer to 'keyed lookup' instead of
'flattened objects'. I pulled most tests into a new flattened module
(although some still need to be migrated).

[elasticmachine]

Pinging @elastic/es-search

[jpountz]

The approach works for me. I'm not seeing this functionality as something that will be used by any other field, so I'd have a slight preference for replacing MappedFieldType#supportsKeyedLookups() with a marker interface, and then moving keyedFieldType to that interface, in order to not add more API surface area to MappedFieldType.

[jtibshirani]

Thanks @jpountz for taking a look. I understand the concern around not adding too many methods to FieldMapper -- using a marker interface works for me too. It doesn't sound like there are big objections from you or @jimczi in terms of the approach, so I'll plan to clean up this prototype.

One note for context: I thought there may be a use case for this lookup functionality outside of flattened fields, but after digging into the details with the other developers I was consulting with, we agreed it is not a good fit. So for the near future flattened fields will be the only consumer of the new marker interface.

[jtibshirani]

@elasticmachine run elasticsearch-ci/1
@elasticmachine run elasticsearch-ci/oss-distro-docs