GH-50690: [C++] Migrate ObjectWriter users to JsonWriter

[Reranko05]

Rationale for this change

This PR continues the simdjson migration by replacing the remaining ObjectWriter users with JsonWriter. With all usages migrated, the obsolete ObjectWriter implementation and its associated build configuration are removed.

What changes are included in this PR?

• Replace ObjectWriter with JsonWriter in:

  • key_material.cc
  • key_metadata.cc
  • local_wrap_kms_client.cc
  • file_system_key_material_store.cc

• Remove the unused ObjectWriter implementation (object_writer.cc and object_writer.h).

• Remove object_writer.h from the installed headers.

• Update the Arrow build configuration to stop building object_writer.cc.

• Link Parquet against simdjson::simdjson since it now includes json_writer_internal.h.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

Closes: #50690

• GitHub Issue: [C++] Remove ObjectWriter after JsonWriter migration #50690

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[github-actions]

⚠️ GitHub issue #50690 has been automatically assigned in GitHub to PR creator.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review is ineligible. To be eligible to request a review, you need a paid Copilot license, or your organization must enable Copilot code review.

[pitrou]

+1, thanks for working on this @Reranko05 !