better lightning-loader configuration

[lance6716]

Is your feature request related to a problem?

currently DM's config for lightning loader is not well-designed

Describe the feature you'd like

Regarding the new backend configuration item, I hope it could be optimized.

tidb:
  backend: "tidb"

1. location
   Since this configuration is related to target, I think it can be placed in

target-database:
  host: "127.0.0.1"
  port: 4400
  user: "root"
  password: ""
  security:
    ssl-ca: "dir-placeholer/task-ca.pem"
    ssl-cert: "dir-placeholer/dm.pem"
    ssl-key: "dir-placeholer/dm.key"
  import-mode: "${value}"  # If multiple downstreams (N->N) are to be added in the future, this approach can also continue to be used

2. the value of the configuration item is not easy to understand, for historical reasons Lightning called tidb and local two modes. We can improve it in dm so that new users can understand it better

• tidb/local could be replaced with logical/physical or sql/sst？,looks better to understand

Describe alternatives you've considered

No response

Teachability, Documentation, Adoption, Migration Strategy

No response

[lance6716]

An alternative for feature 1 is we put import-mode under loader/full-phase configuration, that's more reasonable when we think of the functionality, but we should find a global "loader/full-phase configuration" to put this item.

[sunzhaoyang]

I put it under target-database because, if a task allows multiple targets in the future, it may be possible to use different import-mode for different targets

[lance6716]

cc @glorv

[glorv]

I put it under target-database because, if a task allows multiple targets in the future, it may be possible to use different import-mode for different targets

Since the load backend (loader or lightning) is only effect the load phase, I think it's more reasonable to put it under loader. BTW, this config should be a invisible config.
A brief roadmap of this config:
Sprint 6: Support lightning(default) and loader, loader should be only used in some special cases(when lightning has compatible or performance issues).
Sprint 7: Permanently abandon loader and add local (lightning local backend) to load phase.
Sprint 8: DM can automatically choose between tidb or local backend in load phase.

[lance6716]

from the functionality, this field should be put under loader section.

But we should also make sure this field is related to downstream and loader section is referenced by upstream instances.

@glorv