Introduce new product_group metadata tag

What does this MR do and why?

Describe in detail what your merge request does and why.

Introduces new product_group metadata tag that describes what product group the test belongs to (e.g. source_code). This will later be used to identify correct SET counterparts for test failure issues.

Please note that I went with more descriptive product_group instead of originally suggested generic group to eliminate any possible confusion for people new to Quality/Gitlab as group can be used as a term for other unrelated things. For example, we use group as a scope for feature_flag metadata (feature_flag: { name: 'feature_flag_name', scope: :group }).

MR also adds the metadata to all Source Code tests as the first group to trial it on. After the solution is tested and verified, we will gradually roll it out to other product groups.

See https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1284 & https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/682

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