[feature]Hertzbeat custom plugin.

[zqr10159]

What's changed?

Currently Hertzbeat in use , mainly rely on the alert module to notify the user , and then the user to take a number of measures such as sending a request , the execution of sql , the execution of shell scripts , etc. , but can only be manually or webhook to receive the alert information for automated processing , this pr add a plugin module , the module has an interface Plugin, the user can implement this interface execute method to receive Alert as a parameter for customized operations. After writing the code, users only need to package the plugin module, copy it to the /ext-lib folder in the installation directory, restart the main hertzbeat program, you can realize the custom function of executing the alert, without having to re-package and deploy the whole program.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[crossoverJie]

Gread job, I left a few comments so we can discuss it.

[tomsun28]

👍LGTM

[crossoverJie]

Gread job, LGTM.

Some documentation will need to be added later.