Add support for scanning embedded assoc

[Fs02]

Scan query result to embedded has one or belongs to association when the returned query result have field path to the association (example: "assoc_name.field_name")

example:

var users []Users
err := repo.FindAll(ctx, &users, rel.Select("*", "follower_stats.followers_count").Join("follower_stats").Where(where.Gt("follower_stats.followers_count", 100)))

related: #287

[codecov]

Codecov Report

Merging #289 (59b0b1b) into master (115ffc0) will decrease coverage by 0.21%.
The diff coverage is 81.25%.

@@             Coverage Diff             @@
##            master     #289      +/-   ##
===========================================
- Coverage   100.00%   99.78%   -0.22%     
===========================================
  Files           33       33              
  Lines         2778     2789      +11     
===========================================
+ Hits          2778     2783       +5     
- Misses           0        3       +3     
- Partials         0        3       +3     

Impacted Files	Coverage Δ
association.go	98.16% <66.66%> (-1.84%)	⬇️
document.go	98.57% <84.61%> (-1.43%)	⬇️

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[Fs02]

better implementation: #290