When to install exporter service required dependency/resource?

On branch `/dev/refactor-multiple-exporter-support`

The current install function of `BaseExporter` class is:

```python
class BaseExporter:
def install(self) -> bool:
"""Install the exporter."""
logger.info("Installing %s.", self.exporter_name)
config_success = self.render_config()
service_success = self.render_service(str(self.charm_dir), str(self.exporter_config_path))
if not (config_success and service_success):
logger.error("Failed to install %s.", self.exporter_name)
return False
systemd.daemon_reload()

# Verify installed
if not (self.exporter_config_path.exists() and self.exporter_service_path.exists()):
logger.error(f"{self.exporter_name} is not installed properly.")
return False

logger.info("%s installed.", self.exporter_name)
return True
```

and the timing to trigger this install function is on

```python

def get_exporter(exporter: BaseExporter):
"""Install and get an exporter"""
if exporter.exporter_service_path.exists():
return exporter
installed = exporter.install()
if not installed:
logger.error(f"Failed to install {exporter.exporter_name}")
return
return exporter

class HardwareObserverCharm(ops.CharmBase):
def __init__(self, *args: Any) -> None:
...
self.exporters = []

hardware_exporter = exporter_helpers.get_exporter(
HardwareExporter(self.charm_dir, self.model.config)
)
if hardware_exporter:
self.exporters.append(hardware_exporter)
...

```

The logic only include the systemd service config file generate, which is because we hide the `prometheus-hardware-exporter` dependency in charm's packaging, which is working fine.

But this become a issue when now we need to include more exporter services which are not a python package. There is no hook we can use to handle the install process of the required resource/dependency for exporter.