feat(health): Custom health checks

[gazorby]

Closes #174

It also brings AbstractHealthCheck to register custom health checks:

class MyHealthCheck(AbstractHealthCheck):
    name: str = "my_health_check"

    def ready(self) -> bool:
        return True

...

from starlite_saqlalchemy.sqlalchemy_plugin import SQLAlchemyHealthCheck
from starlite_saqlalchemy.health import AppHealthCheck

config = init_plugin.PluginConfig(health_checks=[AppHealthCheck, SQLAlchemyHealthCheck, MyHealthCheck])
Starlite(route_handlers=[], on_app_init=[init_plugin.ConfigureApp(config=config)])

Calling the health endpoint now returns all health checks statuses:

{
  "app": {
    "BUILD_NUMBER": "",
    "CHECK_DB_READY": true,
    "CHECK_REDIS_READY": true,
    "DEBUG": false,
    "ENVIRONMENT": "prod",
    "NAME": "my-starlite-saqlalchemy-app"
  },
  "health": {
    "app": true,
    "db": true,
    "my_health_check": true
  }
}

This would would also help getting #213 merged since we could easily disable SQLAlchemyHealthCheck if sqlalchemy is not installed.

[sonarcloud]

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[peterschutt]

very nice!