Migrate JWT Signing Algorithm from RSA to ECDSA

[hwupathum]

Thunder currently signs JWTs using RSA, which is secure and widely supported but relatively slow, especially at higher concurrencies.

ECDSA offers comparable classical security with much better signing performance and smaller keys and signatures. This can improve throughput and reduce latency.

Note that neither RSA nor ECDSA is post-quantum secure. We will be anyway introducing quantum secure algorithms in the future for that.