Adds an option for enabling JWT auth to the gateway

[cameronvoell]

Remaining:

• Update README.md with steps for testing JWT authentication

[macroscopeapp]

The exp check after jwt.ParseWithClaims is redundant and can mislabel other validation errors as expiration. Consider removing the post-parse ExpiresAt check; jwt.WithExpirationRequired() already validates it.

-        if time.Now().After(claims.ExpiresAt.Time) {
-            return gateway.Identity{}, gateway.NewPermissionDeniedError(
-                "token expired",
-                ErrTokenExpired,
-            )
-        }

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