FieldProp: add `join` option to combine sub-struct to main struct

[wenchy]

📌 The join option implementation should be based on PR #123.

The join concept is borrowed from SQL join:

A JOIN clause is used to combine rows from two or more tables, based on a related column between them.

Sheet Chapter

ID	Name	SectionID	SectionDesc
map<uint32, Chapter>	string	map<uint32, Section>|{join:"Section.ID"}	string
Chapter’s ID	Chapter’s name	Section’s ID	Section’s Desc
1	Chapter1	101	SectionDesc1
1	Chapter1	102	SectionDesc2
1	Chapter1	103	SectionDesc3
2	Chapter2	201	SectionDesc4
2	Chapter2	202	SectionDesc5

Sheet Section

ID	Name
map<uint32, Section>	string
Section’s ID	Section’s Name
101	Name1
102	Name2
103	Name3
201	Name4
202	Name5