Support context based transaction for create/update only

[gaurav-vmware]

MR details -

Changes for exposing transactions outside datastore CRUD methods.

• Currently, transaction support is only available to create/upsert methods.
• Update/Delete/Find still doesn't support them due to chaining of the
  where clauses

Git issue link - #49 (comment)

[jeyhun]

I'd like to see new/existing unit tests use the new code, in order to be able to see how it works in action and step through the code with a debugger. Right now no unit test touches the line tx = ctx.Value(authorizer.TransactionContextKey("transaction")).(*gorm.DB).Clauses(clause.Locking{Strength: "UPDATE"}) in database.go, which seems like the biggest change in this PR

[krishnamiriyala]

@gaurav-vmware overall looks good to me, add unit-tests and remove unused code.

[codecov-commenter]

Codecov Report

Merging #48 (da43b12) into main (7b623b2) will decrease coverage by 0.10%.
The diff coverage is 48.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   70.42%   70.33%   -0.10%     
==========================================
  Files          12       13       +1     
  Lines        1454     1483      +29     
==========================================
+ Hits         1024     1043      +19     
- Misses        337      347      +10     
  Partials       93       93              

Flag	Coverage Δ
unit-tests	70.33% <48.61%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pkg/authorizer/transaction.go	0.00% <0.00%> (ø)
pkg/datastore/database.go	57.14% <52.63%> (+1.85%)	⬆️
pkg/datastore/helper.go	55.48% <100.00%> (+0.28%)	⬆️

done