Explicit Association Autosave

[Fs02]

!!! This is a breaking changes.

• Previously, any insert or update will automatically save it's association if not empty, this changes will disable that feature by default, and if needed, it can be enabled by setting struct tag autosave:"true".

  type User struct {
    ID        int
    Name      string
    CreatedAt time.Time
    UpdatedAt time.Time
  
    // autosave disabled by default, it's not make sense to update transaction when updating user
    Transactions []Transaction
  
    // enable autosave, might make sense when the association is tightly coupled.
    Address Address `autosave:"true"`
  }

• Autosave will still work with Cascade(true). If cascade enabled, it won't go deeper than autosave, and if cascade disabled, all autosave will be ignored.
• Autosave also applies to Delete operation.
• Not all association support autosave (for example: many to many Support association using join table such as Many to Many #104), and this explicitness allow REL to validate the use of autosave and tell the user explicitly if autosave is not supported in selected association mode.

[codecov]

Codecov Report

Merging #127 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #127   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           54        54           
  Lines         3630      3641   +11     
=========================================
+ Hits          3630      3641   +11     

Impacted Files	Coverage Δ
map.go	100.00% <ø> (ø)
association.go	100.00% <100.00%> (ø)
repository.go	100.00% <100.00%> (ø)

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