Add JSON schema generation (#2005)

* Add JSON schema generation for pyproject.toml

* Generate maturin JSON schema

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add JSON schema generation as a hidden CLI command

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Cargo.toml

@@ -130,6 +130,10 @@ keyring = { version = "2.3.2", default-features = false, features = [
 ], optional = true }
 wild = { version = "2.1.0", optional = true }
 
+# JSON schema
+schemars = { version = "0.8.16", optional = true }
+pretty_assertions = { version = "1.3.0", optional = true }
+
 [dev-dependencies]
 expect-test = "1.4.1"
 indoc = "2.0.3"
@@ -158,6 +162,8 @@ upload = [
  "dep:dirs",
 ]
 
+schemars = ["dep:schemars", "dep:pretty_assertions"]
+
 # keyring doesn't support *BSD so it's not enabled in `full` by default
 password-storage = ["upload", "keyring"]