Use lazy loading

[chetanchilhate]

Summary of change

Lazy load authors in book and books in author

Related issue

Closes #20

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[chetanchilhate]

@knjk04 let me if any change is needed

[chetanchilhate]

@knjk04 added test cases have look while writing test cases, I found one issue with Book constructor we are checking not null we should also check list should not be empty if does we should throw IllegalArgumentException book must have at least one author

[chetanchilhate]

@knjk04 I have made suggested changes have look let me know if I missed anything I will fix it