types/upload: support custom upload timestamps from native API docs

[goanpeca]

Summary

The current native API docs expose custom upload timestamp support, but the SDK does not model it for normal uploads or large-file starts.

Evidence

• b2_start_large_file request includes customUploadTimestamp.
• b2_upload_file documents the X-Bz-Custom-Upload-Timestamp header.
• SDK StartLargeFileRequest lacks customUploadTimestamp.
• SDK UploadFileHeaders and high-level upload options do not expose the custom timestamp header.

Impact

Consumers cannot set the documented custom upload timestamp through typed SDK APIs.

Acceptance criteria

• Add customUploadTimestamp to the large-file start request type and raw call path.
• Add typed support for X-Bz-Custom-Upload-Timestamp on upload-file headers/options.
• Decide how this relates to existing lastModifiedMillis metadata convenience.
• Add simulator and unit coverage.

Versioning note

This can be additive if implemented as a new optional field/header, so it should not require a v1.0 breaking surface change.

[goanpeca]

Implementation direction update:

This remains good v0.2.0 additive work. Add customUploadTimestamp as an optional typed field/header without changing the existing lastModifiedMillis convenience behavior. Where both concepts are present, document that customUploadTimestamp maps to B2's dedicated upload timestamp API/header and lastModifiedMillis remains file-info metadata.

Simulator note: simulator upload and start-large-file paths should accept and persist the canonical custom upload timestamp behavior once the SDK exposes it.