Make more hardcoded values configurable

### Proposal
There are several hard-coded values that control nomad agent behavior that we'd like to adjust. It would be nice if we could expose config options for these values.

I wanted to get a sense for whether or not this is something maintainers would be happy with. If it is, I'm happy to submit PRs to add the knobs that we care about.

### Use-cases
Here are some values we'd like to adjust, but this list is non-exhaustive and likely to grow:
- `registerRetryIntv` https://github.com/hashicorp/nomad/blob/cf8dde0850a23d4e8b0b48bc5fea2f66388ab711/client/client.go#L83
- `initialHeartbeatStagger` https://github.com/hashicorp/nomad/blob/cf8dde0850a23d4e8b0b48bc5fea2f66388ab711/client/client.go#L106

Start by reading the hard-coded values in client/client.go, including registerRetryIntv and initialHeartbeatStagger. The proposal does not define the full set of values or an agreed configuration interface, so the scope and acceptance criteria need maintainer clarification before implementation; done would mean the agreed agent behaviors are configurable.

Written by the indexing model from the issue text.