feat: expose validate() as abstract

[alexey-pelykh]

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: typing enhancement

What is the current behavior?

extends PassportStrategy(Strategy) does not inform developer that validate must be implement and about its signature.

What is the new behavior?

extends PassportStrategy(Strategy) informs developer that validate must be implement and about its signature.

Does this PR introduce a breaking change?

• Yes
• No

In case there are abstract classes that derive from PassportStrategy(Strategy) that didn't mark validate() as abstract, those would issue a build error now.

Other information