feat(common): Allow adding custom validators to ParseFilePipeBuilder

[buddh4]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Currently it is not possible to add custom file validators using the ParseFilePipeBuilder.

What is the new behavior?

This PR adds an addValidator(validator: FileValidator) function, which can be used to add additional validators.

Does this PR introduce a breaking change?

• Yes
• No

Other information

My use-case for this feature is adding a file-type based mime validator. This change would also allow extending the ParseFilePipeBuilder class with custom builder functions.

[coveralls]

Pull Request Test Coverage Report for Build cf05d90e-9a25-4cfc-8b1d-d781e20d5774

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.788%

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Totals
Change from base Build 0e7c875e-a014-419b-8b47-f5f8dfb3a71e:	0.0%
Covered Lines:	6115
Relevant Lines:	6520

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💛 - Coveralls

[kamilmysliwiec]

cc @thiagomini 👀

[thiagomini]

LGTM! This is an addition I've been thinking about for a while, thanks for the contribution!

[kamilmysliwiec]

lgtm