DataStreamDownload API improvements

[corbin-phipps]

Type

• Bug fix
• Feature addition
• Feature update
• Documentation
• Build Infrastructure

Side Effects

• Breaking change
• Non-functional change

Goals

Address PR comments.

Technical Details

• Simplify oneof field names in Properties.
• Use const std::string& instead of const std::string.
• Add class member for client reactor timeout values.
• Re-order message fields so always present fields are first.
• Allocate DataStreamReader on stack in test client code.

Test Results

All tests pass.

Reviewer Focus

None.

Future Work

None.

Checklist

• Build target all compiles cleanly.
• clang-format and clang-tidy deltas produced no new output.
• Newly added functions include doxygen-style comment block.