Expose underlying DBAdapter interface for custom implementations

[VinGarcia]

Adds and exposes the DBAdapter interface.

This interface abstracts away our dependency on the database/sql library and allows we ourselves and our users to implement alternative implementations if necessary.

This allowed us to add the function NewWithPGX() which instantiates the ksql.Client but using the PGx as the backend database adapter.
This configuration allows the users to use all the query param types by PGX and it also showed great results in the benchmark which is also good.

$ make bench TIME=5s
go test -bench=. -benchtime=5s
goos: linux
goarch: amd64
pkg: github.com/vingarcia/ksql
cpu: Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
BenchmarkInsert/ksql-setup/insert-one-4         	    5293	    960859 ns/op
BenchmarkInsert/pgx-adapter-setup/insert-one-4  	    7982	    736973 ns/op
BenchmarkInsert/sqlx-setup/insert-one-4         	    6854	    857824 ns/op
BenchmarkQuery/ksql-setup/single-row-4          	   12596	    407116 ns/op
BenchmarkQuery/ksql-setup/multiple-rows-4       	   15883	    391135 ns/op
BenchmarkQuery/pgx-adapter-setup/single-row-4   	   34008	    165604 ns/op
BenchmarkQuery/pgx-adapter-setup/multiple-rows-4         	   22579	    280673 ns/op
BenchmarkQuery/sqlx-setup/single-row-4                   	   10000	    512741 ns/op
BenchmarkQuery/sqlx-setup/multiple-rows-4                	   10779	    596377 ns/op
PASS
ok  	github.com/vingarcia/ksql	94.951s
Benchmark executed at: 2021-08-01
Benchmark executed on commit: 37298e2c243f1ec66e88dd92ed7c4542f7820b4f

Other changes

• Rename structs/ subpackage to kstructs in order to avoid name collisions with packages with similar names.
• Rename ksql.SQLProvider interface to ksql.Provider in order to avoid stuttering
• Rename ksql.MockSQLProvider struct to ksql.Mock in order to avoid stuttering
• Fixed a few bugs

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This discussion was created from the release Expose underlying DBAdapter interface for custom implementations.