Change signingRoot to signing_root

## Proposed Changes
Consider changing `signingRoot` property to `signing_root` so that it is consistent with other properties in the spec. The current remote signer implementation of these spec should be backward compatible till all validator clients start sending signing_root.

- https://github.com/ethereum/remote-signing-api/blob/8e294a901f5c17f767893d0fd2190ce6095652a7/signing/schemas.yaml#L19
- https://github.com/ethereum/remote-signing-api/blob/8e294a901f5c17f767893d0fd2190ce6095652a7/signing/schemas.yaml#L100
- https://github.com/ethereum/remote-signing-api/blob/8e294a901f5c17f767893d0fd2190ce6095652a7/signing/schemas.yaml#L196

## Target version
v2.0.0

The referenced schema entries are in signing/schemas.yaml at lines 19, 100, and 196; inspect those definitions first. Rename signingRoot to signing_root for the v2.0.0 specification and verify the stated backward-compatibility behavior for clients that still use the old name.

Written by the indexing model from the issue text.