[httpjson] Add value_type parameter to httpjson transforms

[marc-gr]

What does this PR do?

Adds value_type parameter to the transforms.
Adds mul and div template functions.
Modifies toInt to accept more types other than string

Why is it important?

This allows to use the output from the value templates as other value types different than string. Initially int and json types have been added, allowing to define the output of the templates to be used as integer literals eg 0 vs "0" or JSON objects {"foo":"bar"} or ["foo","bar"} vs "{\"foo\":\"bar\"}" or "[\"foo\":\"bar\"]".

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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• Start Time: 2021-07-13T07:00:13.708+0000

• Duration: 91 min 44 sec

• Commit: 565a33b

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Passed	7863
Skipped	1201
Total	9064

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Test	Results
Failed	0
Passed	7863
Skipped	1201
Total	9064