Support automatically reload when config file changes

[J1a-wei]

We are developing a eth staking infrastructure, using web3signer for signatures, and it looks great. It has been running stable for six months in mainnet. There is a small feature that we hope to implement, since our environment is in k8s. When we have a new user staking eth, we will modify the configmap and the final application time to the container is uncertain. We have to poll to call the interface that queries the load key and call /reload manually. So is it possible to listen to the configuration file via fsnotify(I am not a Java expert, this is a Go -based open source library) or some technique that allows web3signer to automatically reload when changes occur?

Relevant issue: #689
Kubernetes: https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/
configMapAndSecretChangeDetectionStrategy

[iamhsk]

Thank you for opening this issue. We are currently working on Shanghai/Capella upgrade, so after those upgrades we'll prioritize this issue accordingly. But in the meantime if you had other feedback on web3signer, please feel free to reach out on Discord or GitHub.

[iamhsk]

Related to #512