Add support for parsing boolean strings

[sandratatarevicova]

Description

class-transformer is often used in NestJS projects to parse request bodies/params. The current version transforms the string 'false' to true and requires ugly workarounds to make it work as expected, see #626, https://stackoverflow.com/questions/59046629/boolean-parameter-in-request-body-is-always-true-in-nestjs-api an the other linked issues.

This PR adds configuration option parseBooleanStrings, which allows parsing boolean strings. It is disabled by default so that it does not break existing projects.

Checklist

• [ x ] the pull request title describes what this PR does (not a vague title like Update index.md)
• [ x ] the pull request targets the default branch of the repository (develop)
• [ x ] the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• [ x ] tests are added for the changes I made (if any source code was modified)
• [ x ] documentation added or updated
• [ x ] I have run the project locally and verified that there are no errors

Fixes

fixes #626
fixes #2170
fixes #306