Update jwk validation logic

[leelalagudu]

Issue #, if available: Fixes #6

Description of changes:

• Consider alg as an optional field on the JWK while validating JWK
• Evaluate if kty on the JWK is set to RSA
• If JWK has alg set, then validate that alg set in JWT header matches the alg element of JWK
• Validate that JWT alg header is set to RS256
• Update jest version

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