fix: typings for PassportStrategy

[NickBolles]

Return the instance type instead of the class type, this allows for intellisense to find the authenticate and other functions

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) - n/a
• Docs have been added / updated (for bug fixes / features) - n/a

PR Type

What kind of change does this PR introduce?

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

this could almost be an enhancement though...

What is the current behavior?

Issue Number: N/A

What is the new behavior?

correct intellisense on strategy services, especially in unit tests

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Thank you!