Feat: Custom request body for Webhook Notifications

[chakflying]

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Description

Basically a continuation of #751, but only for Webhook notifications.

There is a trend of people using the Webhook notification type for other notification providers, but the body does not conform to the need of specific providers, therefore errors are encountered (#3087, #3084, #2967, #2960, #2845, #1160, #949). Allowing users to define a custom body would make Webhook actually useful.

This still doesn't solve the case for variables in query string tho. (#2813)

Type of change

• New feature (non-breaking change which adds functionality)

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 generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

default	custom

[CommanderStorm]

I have left some small inline comments

[louislam]

Trying to test it using http://localhost:3001/test-webhook and submit it without content-type.

• By default it seems a little bit weird to me, as it is using x-www-form-urlencoded and the json string is inside the first object key with empty string value.

[chakflying]

The "form-urlencoded" default is provided by axios. I can override this default to something like text/plain or application/json, but I'm not sure if there will be unintended consequences.

The strange body is probably because express is attempting to parse it into a javascript object. I use a rust program to print the body as string and it works fine.

[chakflying]

Express parses fine if you follow the format of form-urlencoded (ignore the unicode):

{
  accept: 'application/json, text/plain, */*',
  'content-type': 'application/x-www-form-urlencoded',
  authorization: 'Bearer alkdsfhgalowefvn',
  'user-agent': 'uptime-kuma/1.21',
  'content-length': '33',
  host: 'localhost:3001',
  connection: 'close'
}
{ 'Test Monitor': '[Test Monitor] [✅ Up] OK' }

[louislam]

I got your point, I somehow wrongly thought the custom body must be a json due to the example in the placeholder.