Fix ECDSA signature size

Correctly serializes ECDSA signature R / S values according to elliptic curve.

Before this patch, the code assumes R / S values will be 32 bytes. This is true for P-256 and P-384 keys, but is not true for P-521 keys. P-521 keys result in 66 byte R / S values.

From https://tools.ietf.org/html/rfc7515#appendix-A.3:

Section on ES256

We need to split the 64 member octet sequence of the JWS Signature

Section on ES512

We need to split the 132-member octet sequence of the JWS Signature into two 66-octet sequences

Note that the curve name (i.e., P-521) is different than the JWT algorithm name (i.e., ES512). The 512 in ES512 refers to SHA512. Also note that P-521 != 512, that's not a typo.

In order to support the different key curves dynamically, the signer now gets the curve name from the key during signing and verification. This curve is used to determine how wide the R / S values should be.

Remove OpenSSL

Removes OpenSSL and migrates calls to our own vendored BoringSSL (#14, #13).

BoolClaim handles strings

BoolClaim wasn't properly handling "true" and "false" as strings.

Apple/Google identity tokens

Adds commonly used AppleIdentityToken and GoogleIdentityToken structs. This includes code necessary to validate the apple/google provided identity data against their respective JWKS elements.

This patch also restructures the project a bit to move classes into their own files with hierarchical folders to make it easier to find the items you're looking for.

JWKS.find

Adds new methods for finding JWKs from by identifier and type (#10).

let jwks: JWKS = ...
let signInWithApple = jwks.find(identifier: "AIDOPK1", type: .rsa)

Fix Payload Verification

JWT verification using JWTSigner and JWTSigners now correctly invokes JWTPayload's optional verify method. (#6)

JWTKit 4.0.0 Beta 2.1

• Replaced all fatalErrors with Swift errors.

JWTKit 4.0.0 Beta 2

• Separated from vapor/jwt into new repo
• Removed JWT type and split functionality into JWTSigner and JWTSigners
• New internal JWTParser and JWTSerializer types to reduce code dupe