fix: pad elliptic curve coordinates correctly in JWK representation

[SalladinBalwer]

When creating and using JWK representations the byte array is padded at the start with 0x00 to ensure that all hashes of the JWK will be correct.

When calculating the hashed token in parse.go and when creating proofs in create.go the FillBytes(buf []byte) function is used on the big.Int to get the byte array representation with added zeros at the start.

• Fixes [Bug]: Invalid JWK thumbprint when leading zeros in JWK coordinate #21

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