Add xml support to HTTP monitors

[bayramberkay]

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Description

There is an existing pr request but I guess it hasn't been active for a long time. That's why I just wanted to make a pr request by copy-pasting the xml part. I think it will help most people waiting for this feature.

Fixes #(issue)

#1679
#1332
#1987
#2497

Type of change

Please delete any options that are not relevant.

• 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)

[bayramberkay]

@louislam , @JustinTisdale commits are protected as we speak. Can we go live?

[chakflying]

Should users be able to override the contentType by setting custom headers?

[louislam]

Should users be able to override the contentType by setting custom headers?

Custom headers should be the higher priority.

[Genc]

Should users be able to override the contentType by setting custom headers?

Custom headers should be the higher priority.

How should we continue? Is the current state enough?

[Genc]

How can we advance the process this pr? @louislam @chakflying

[Genc]

How should we continue the process as there are some friends waiting for this? @louislam @chakflying

[louislam]

Found 2 issues:

1. Body Encoding will be empty for old monitors
   

2. The example placeholder is still in JSON format.
   

[Genc]

Found 2 issues:

1. Body Encoding will be empty for old monitors
   
2. The example placeholder is still in JSON format.
   

Thank you very much for your attention. I have good news, fixed them! @louislam

[danwie]

@Genc Could you please explain why Body Encoding: "Form" was removed in #2610 commit 3adc9e6 ? How are we supposed to make a ordinary POST (x-www-form-urlencoded) when the only options are Json/XML?

[louislam]

@Genc Could you please explain why Body Encoding: "Form" was removed in #2610 commit 3adc9e6 ? How are we supposed to make a ordinary POST (x-www-form-urlencoded) when the only options are Json/XML?

Some History:

Originally xml/form data implementation was started by #1987, but it was abandoned and unfinished for a long time. Then @bayramberkay and @Genc wanted to implement xml and used #1987's code.

The form data implementation is never existed in any previous versions of Uptime Kuma. The commit 3adc9e6 was just removing those unfinished code only.

[anderskvist]

I have created this PR that should be ready to merge that will add x-www-form-urlencoded requests - #3499