[DSIP-92][Alert] Support DolphinScheduler Configuration Markdown and String Alert Template

### Search before asking

- [x] I had searched in the [DSIP](https://github.com/apache/dolphinscheduler/issues/14102) and found no similar DSIP.

### Motivation

I want use dolphinScheduler configuration markdown or string alert template, like is:

~~~
markdown
## ${alertSubject}
* project name: ${projectName}
* workflow name: ${workflowName}
* alert time: ${alertTime}
* alert status: ${alertStatus}
~~~
or
~~~
string
${alertSubject}, project name: ${projectName}, workflow name: ${workflowName}
~~~

But this feature was not found.

### Design Detail

In the alert instance editor, if you select the any plugin (Need plugin support) and choose Markdown or String as the message type,If it is a string, it will be the same as before, if markdown need a larger text box will appear below. Use this box to enter your Markdown template. You can use JSON strings (System built-in: like ${alertSubject}, ${projectName}, ${alertTime} etc.) to dynamically replace values within the template.
like is:

### Compatibility, Deprecation, and Migration Plan

This feature is fully backward-compatible and introduces Markdown, string template substitution. Data returned to any plugin will be automatically parsed for formatting.

### Test Plan

Functional testing

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start with the alert instance editor and the alert plugin support described in the design, then trace how alert messages are passed to plugins. Done means Markdown and string templates can be configured, built-in values such as ${alertSubject} and ${projectName} are substituted, existing behavior remains compatible, and functional testing covers the feature.

Written by the indexing model from the issue text.