feat: Allow private/public key to be override when signing/verifying

[maitrungduc1410]

This allows you to pass the privateKey property in the options of sign, signAsync and publicKey in the verify, verifyAsync. It does not override a secretOrKeyProvider though only the privateKey and publicKey passed into the JwtModule.

Resolves #302

PR Checklist

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• Tests for the changes have been added (for bug fixes / features)
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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?

Currently there's no way to override privateKey and publicKey passed in from the module. This makes it difficult in case of we want to use multiple keys to sign and verify JWT token. Example: in a multi-tenant app, each tenant will have its own private/public key to sign its user's token.

Issue Number: 320

What is the new behavior?

It's now possible to override privateKey and publicKey from the module when signing/verifying token.

Does this PR introduce a breaking change?

[ ] Yes
[x] No