mysql: fix dispose-method not called in external-mode

[gabor]

this change ensures that the Dispose() method is called both in external-mode and core-mode.

changes:

1. added CheckHealth to the sql_engine.go structure (otherwise there is no CheckHealth in external-mode)
2. moved all the ProvideService things into mysqL_service.go (added a forward for CheckHealth)

how to test: basically, we need to see the debug-level log-lines Disposing DB... and DB disposed, in both core and external mode. so, make sure the grafana you use has log-level-debug enabled, something like this in the config:

[log]
level = debug

1.as core plugin (the default behavior)

• just run it as always (yarn start and make run)
• verify that you are running mysql in it's normal mode
  • go to plugins, choose mysql. verify that in the signature section it says core
• make sure the mysql datasource plugin still works (just run a query like select 4*5 in explore)
• go to the datasource config page, press [save & test]
• clear the terminal where the logs are shown
• press [save & test] again
• verify that after a couple seconds the required log-lines appear (Disposing DB... & DB disposed) (these will appear after a short delay, not immediately)
  2. make sure it runs as an external plugin. add this to the grafana-config:

     
     [feature_toggles]
     externalCorePlugins = true

[plugin.mysql] as_external = true

- build the plugin
        - run `make build-plugin-go PLUGIN_ID=mysql` . this will generate the "backend" binaries in `public/app/plugins/datasource/mysql/dist`
        - run `yarn workspace @grafana-plugins/mysql build`. this will generate the "frontend" files in `public/app/plugins/datasource/mysql/dist`
        - at this point all the needed files are in `public/app/plugins/datasource/mysql/dist`
        - make sure to copy/move the `dist` files to somewhere where your grafana expects plugin-files (i think in the usual devel grafana situation it's the `path/plugins` folder.
- run grafana
- go to `plugins`, choose `mysql`. check what it says under `signature`. it should be `unsigned`, not `core`.
- do the same steps as we did in the core-mode (verify sql works, verify `[save&test]` shows the required log-lines)

[gabor]

/deploy-to-hg

[ephemeral-instances-bot[bot]]

• Preparing your instance. A comment containing your instance's url will be added to this PR when the instance is ready.
• Your instance will be ready in ~10 minutes. Follow the workflow progress
• Slack channel: #proj-ephemeral-hg-instances
• Building instance with gabor/mysql-experiment oss branch and main enterprise branch. How to choose a branch

[ephemeral-instances-bot[bot]]

• Your instance can be accessed at: https://ephemeral1511182187330gabor.grafana-dev.net
• The instance is not using the CDN assets.
• How to access / How to update instance config / How to build a specific branch