Added public key support in JWT authentication scheme

Proposed changes

This change set allows developers to provide public key through options in JWT authentication scheme and it also allows JWT verification via public key as well. This pull request resolves the following issues
Issue-122 Issue-130

Types of changes

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[ ] Bugfix (non-breaking change which fixes an issue)
[x ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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[ ] I have added tests that prove my fix is effective or that my feature works.
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Further comments

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adonisjs / auth