[SPARK-51702] Revise sparkSession/read/write/columns/schema/dtypes/storageLevel API

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to revise sparkSession/read/write/columns/schema/dtypes/storageLevel API to resemble the other language clients' APIs.

Why are the changes needed?

To provide a consistent UX to the users.

Does this PR introduce any user-facing change?

• Change func to var like the following.

- public func schema() async throws -> String {
+ public var schema: String {
+   get async throws {

• Add public explicitly.
• Revise test cases accordingly.

How was this patch tested?

Pass the CIs and manual review.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Hi, @viirya . After your review comments, I revisited and revised similar patterns in the existing APIs in order to provide better consistent UX. Could you review this when you have some time?

• [SPARK-51693] Support storageLevel for DataFrame #38 (comment)

[dongjoon-hyun]

This is renamed to avoid a conflict with public var schema.

[dongjoon-hyun]

Thank you, @viirya . Merged to main.