Resolve Issue #5724: parse all content types ending with +json as json in rest data source

[nholik]

This commit addresses issue 5724. It is a simple change to treat all content types ending with +json similar to how all content types beginning with application/json are automatically parsed as JSON. The helps cover some other common types of JSON types beyond the original hard coded exception.

[apollo-cla]

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[glasser]

This looks good. It needs a CHANGELOG.md entry before it can be merged, which I can get to soon unless you do it first. (It's a little weird how we still have a single CHANGELOG.md file even though packages such as apollo-datasource-rest have unrelated versioning; I'd like to eventually move apollo-datasource-rest and apollo-datasource to its own repo as part of improving this.)