feat: add dynamic secret key provider function and tests

[tomsiwik]

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
[x] 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?

There is no way to assign a secret dynamically.

Issue Number: #26

What is the new behavior?

With secretOrKeyProvider you can assign a secret or public/private key for each sign/verify request separately.

Does this PR introduce a breaking change?

[x] Yes
[] No

Downwards compatibility exists for secretOrPrivateKey. However a warning will be displayed if not using the explicitly named options (priority based options):

1. secretOrKeyProvider
2. secret
3. publicKey and privateKey

Other information

Hope this time I don't constantly race against renovates tasks.

[tomsiwik]

@PachowStudios : I just cleaned up this PR so you guys are also aware (I somehow messed up my rebase.. fool me).

[shayded-exe]

Awesome, thank you for this!

[kamilmysliwiec]

Thank you for your contribution again. I'll publish this shortly.