feat(pg_dump): Add '--if-exists' option for pg_dump

[DmytroLitvinov]

Description

Add option --if-exists for pg_dump command. Resolves #478

Checklist

Please update this checklist as you complete each item:

• [ ] Tests have been developed for bug fixes or new functionality.
• [X] The changelog has been updated, if necessary.
• [X] Documentation has been updated, if necessary.
• [X] GitHub Issues closed by this PR have been linked.

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