Investigate Key Switch In Low Latency Environment

Sync with Execution and Solo Team on Feasibility, and implement

Notes from discussion on topic:

```
Context: Consensus team found that switching from RSA to elliptic curve keys (ED25519) improves latency in low-latency environments. Verification is the bottleneck (~70μs for RSA vs much faster for EC).
Key Findings from Discussion:

Three elliptic curve options (ED25519, ECDSA, BLS/Schnorr) are all roughly equivalent in performance and all significantly faster than RSA
Joseph recommended considering ECDSA for Schnorr compatibility (relevant to TSS/threshold signatures)
In a hashsphere/low-latency environment, this is straightforward — just issue EC certificates instead of RSA
On mainnet, it would require re-keying the network (not planned for Q2)
With TSS (Threshold Signature Scheme) coming, block signing will use its own keys (re-keyed nightly), separate from event signing keys in the roster
RSA keys currently used for state signing will eventually be replaced by TSS

Impact on Teams:

DevOps/RE need to be aware — Solo provisioner would need to support creating EC certificates for spheres
Execution team should be consulted on TSS key types for alignment
Dan (DevOps) confirmed no significant DevOps work needed for running ED25519 in non-mainnet environments
This change is scoped to low-latency environments only — not intended for mainnet
```

No files, tests, or entry points are named. Start by syncing with the Execution and Solo teams, then inspect the Solo provisioner's certificate-creation path and confirm the TSS key-type requirements. Done means an agreed low-latency-only approach and support for EC certificates for spheres, without extending the change to mainnet.

Written by the indexing model from the issue text.