Update HTML parser to pick up preload attribute for rel

[matthewt-assurity]

Description

Updated JMeter HTML parsers to pick up on the preload value for the rel attribute in <link> tags.

A constant has been added to HTMLParser.java for the preload value. The parser implementation classes have been updated to use this constant.

Motivation and Context

During a project we found some embedded resource files that were not being downloaded by default. To address this in the meantime we have used the parallel sampler extension. We found that the problem was the preload value wasn't being picked up as a valid value in the JMeter HTML parsers, so I am proposing that this becomes part of the HTML parsers.

I am unaware of any open issue for this.

How Has This Been Tested?

./gradlew runGui has been run, and a simple test hitting an HTML page with rel=preload attributes was run.

./gradlew check has been run and passes.

Types of changes

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

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[matthewt-assurity]

This is my first time contributing to JMeter, is there any documentation that should be updated for this change?

[vlsi]

Do you think you could update https://github.com/apache/jmeter/blob/master/xdocs/changes.xml as well?
I think it is worth adding the improvement in the release notes.

[vlsi]

Merged in 6d6479b