Support synthetic source for fields with normalizers enabled

[martijnvg]

Today synthetic source can't be configured if a mapping contains keyword fields with normalizers. The reason why this is disallowed, is that when documents get reindexed we can't synthesize from doc values because the original value was altered.

In order to support synthetic source when normalizers are enabled, 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)