Support Brainpool curve

Working a lot with German health related projects this would be really handy if it supported Brainpool curved keys.

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The JWT provided is invalid because JSON error: unknown variant `BP256R1`, expected one of `HS256`, `HS384`, `HS512`, `ES256`, `ES384`, `RS256`, `RS384`, `RS512`, `PS256`, `PS384`, `PS512`, `EdDSA` at line 1 column 16
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