Support synthetic source for fields with copy_to feature enabled

[martijnvg]

Today synthetic source can't be configured if a mapping contains fields with copy_to definitions. The reason why this is disallowed, is that when documents get reindexed we often synthesize from doc values and then the documents in the target index could have accuracy loss. A provided field value at index time value may not be the same at reindex time.

In order to support synthetic source when copy_to definitions are defined, we should enforce storing these fields, so that we can synthesize from stored fields and avoid the potential accuracy loss at reindex time.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)