Allow explicit re-render of template

### Proposal
Add new parameter `render_on_task_restart` for `template` block.
When this boolean flag is set to `true`, the template will be re-rendered when a task is restarted.

The flag can be added to the check here.

https://github.com/hashicorp/nomad/blob/b30f76e7d72114f9d06a4ee2c34c7242a1f5572c/client/allocrunner/taskrunner/template_hook.go#L98-L101

### Use-cases
For template retrieving secrets from Vault, due to lack of blocking query in Vault, the template will only re-render when getting closer to the lease duration. This is not ideal when Vault secret is no longer relevant, e.g. lease get revoked.

Having this parameter will enable template to be re-rendered explicitly when the task is restarted.

### Attempted Solutions
The current workaround is to modify jobspec with some unique meta so that new alloc will be created, which means template will be re-rendered.

I can help with the PR if the feature request is accepted.