Add @People feature in DingTalk notifications

[boomboomxx]

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Description

Sometimes we need to notify specified users in a webhook instead of all users in Dingtalk, and right now it only supports @everyone.
According to Doc, we can tell that it supports that feature.
Therefore, this PR adds the feature and adds it to the test message. It supports two ways of specifying:

1. mobile list
2. user ID list

Type of change

Please delete any options that are not relevant.

• User interface (UI)
• Other

Checklist

• [√] My code follows the style guidelines of this project
• [√] I ran ESLint and other linters for modified files
• [√] I have performed a self-review of my own code and tested it
• [√] I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• [√] My changes generates no new warnings
• [√] My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

[CommanderStorm]

Overall, looks like a reasonable change.
I have left a few changes that I would like to see

[boomboomxx]

Overall, looks like a reasonable change. I have left a few changes that I would like to see

I have a question, how do I check the required rules in vue-multiselect.
Do you have any ideas?

[CommanderStorm]

required rules?

[boomboomxx]

required rules?

Like <input required>, the vue-multiselect dose not have required attr.

[boomboomxx]

Hi @CommanderStorm, Please help to review again.
I changed the input to vue-multiselect and changed the data validation rules to do the data is empty compatibility.
Currently there is only one problem, how to check if the value of vue-multiselect is null on submit time