feat(): support for asynchronous version of secretOrKeyProvider

[xzyfer]

PR Checklist

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• 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)
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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?

Issue Number: #235

What is the new behavior?

Allow ability to provide an asynchronous version of secretOrKeyProvider. However, when using asynchronous version of secretOrKeyProvider, synchronous versions of .sign and .verify will log a warning message and throw an exception before calling .sign or .verify.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I'm not sure if this PR contains breaking changes, because synchronous versions of .sign and .verify can throw exceptions as well.

This PR is an updated version of #469 with conflicts resolved and typos fixed.

[xzyfer]

@kamilmysliwiec how likely are we to see progress on this given the original issue #235 been closed and the last submitted PR sat dormant for over 2.5 years?

I ask as this missing capability is a problematic us and anyone needing JWKS support like Auth0 users.

[kamilmysliwiec]

lgtm