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?

Dynamically sending a privateKey to jwtService.sign() (as opposed to using the JwtModule config) does not support passphrase encoded types (inconsistency in the Typescript interface).

Issue Number: N/A

What is the new behavior?

JwtSignOptions now supports passphrase encoded private key (using jwtService.sign()).

Does this PR introduce a breaking change?

• Yes
• No

Other information

jwt.Secret type includes string | Buffer as well as { key: string | Buffer; passphrase: string }.
This change is required in order to enable using a {key,passphrase}-structured privateKey in jwtService.sign() calls like it does in JwtModule.register().
The use case is when you want to dynamically provide such key to the sign method (e.g. when each user/token has a different key pair).