Add GraphQL arguments to filter by custom fields

What does this MR do and why?

Add custom field arguments for work items and issue boards.

This was already added to issues in !183391 (merged)

References

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• Related to #515946 (closed)

MR acceptance checklist

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changed milestone to %17.10

added backend devopsplan groupproject management typefeature workflowin dev labels

assigned to @engwan

added pipelinetier-1 label

added sectiondev label

mentioned in task #515946 (closed)

added documentation label

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	This merge request adds or changes documentation files and requires Technical Writing review. The review should happen before merge, but can be post-merge if the merge request is time sensitive.

Documentation review

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• doc/api/graphql/reference/_index.md (Link to current live version)

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Reviewer roulette

Category	Reviewer	Maintainer
backend	@shreyasagarwal (UTC+1, 7 hours behind author)	@manojmj (UTC+5.5, 2.5 hours behind author)

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removed workflowin dev label

changed the description

@stefanosxan can you do the backend review please?

requested review from @stefanosxan

LGTM!!

approved this merge request

added pipeline:mr-approved label

added pipelinetier-2 label and removed pipelinetier-1 label