feat: add self-extractor for JSON type

[chrxn1c]

Does your PR solve an issue?

No, but i can create one if bureaucracy is expected.

Is this a breaking change?

No. Adding a new method for already existing type is not considered a breaking change for downstream-users.

ThIs PR introduces .into_inner method for Json<T> type, so that users can extract T from Json<T>, if they prefer to do so. Regarding sqlx-core types, there's already .into_inner method in Text type, but there is none in Json type. Using .into_inner might come really handy, especially when iterating and doing .map(Json::into_inner) instead of .map(|Json(x)| x).

Tests

Regarding tests and Text type, its .into_inner method is not found in examples / whatnot. Which is why there are no tests written -- I expected that if another type .into_inner's method does not appear in tests / examples, similar the situation should be for my change. If I'm wrong -- never hesitate to announce that :)

[chrxn1c]

7 timed-out jobs in CI, 1 failed due to

2025-12-11T14:26:27.8248011Z --2025-12-11 14:26:27--  https://github.com/nalgeon/sqlean/releases/download/0.15.2/ipaddr.so
2025-12-11T14:26:27.8494057Z Resolving github.com (github.com)... 140.82.112.3
2025-12-11T14:26:27.8547619Z Connecting to github.com (github.com)|140.82.112.3|:443... connected.
2025-12-11T14:26:29.8759347Z HTTP request sent, awaiting response... 503 Service Unavailable
2025-12-11T14:26:29.8862495Z 2025-12-11 14:26:29 ERROR 503: Service Unavailable.

some Microsoft engineering going on, hold on for a sec, will trigger CI via reopening PR

[chrxn1c]

well, now CI passes, but 2 jobs running on MacOS 13 fail, since

The macOS-13 based runner images are now retired. For more details, see actions/runner-images#13046.

besides that, this PR is ready to be reviewed. deleting macos-13 from matrix doesn't seem too complex, but i don't know whether your team should add macos-14 instead of macos-13, or just leaving macos-latest will do.