Auto populate join fields

[Fs02]

• Extract Metadata methods from Document and Association struct to DocumentMeta and AssociationMeta
• Added Populate callback on query builder
• The callback is now used to generate Join fields
• Later this will be used to populate fields to be scanned (Add support for scanning joined assoc #290)
• Added join.Assoc and join.AssocWith api

[codecov]

Codecov Report

Merging #292 (1709992) into master (f5e5856) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #292   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        35    +2     
  Lines         2802      2857   +55     
=========================================
+ Hits          2802      2857   +55     

Impacted Files	Coverage Δ
association.go	100.00% <100.00%> (ø)
association_meta.go	100.00% <100.00%> (ø)
collection.go	100.00% <100.00%> (ø)
document.go	100.00% <100.00%> (ø)
document_meta.go	100.00% <100.00%> (ø)
join_query.go	100.00% <100.00%> (ø)
query.go	100.00% <100.00%> (ø)
repository.go	100.00% <100.00%> (ø)

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