Add GenericWriter and raw bytes mode for verbatim VRS stream copying

[danielyan86129]

Summary:

• Add PyGenericWriter class that extends vrs::Recordable to write raw bytes
  without predefined DataLayout definitions
• Add createGenericStream() to VRSWriter for creating generic streams by
  numeric RecordableTypeId
• Add raw bytes mode to Reader that captures decompressed record payload
  as raw bytes via processRecordHeader, skipping content block parsing
• Add Python bindings for GenericWriter (create_raw_record, set_tag,
  set_compression, get_stream_id) and Reader (set_raw_bytes_mode,
  get_raw_bytes_mode)
• Add VRSGenericStream Python wrapper and VRSRecord.raw_record_bytes /
  has_raw_record_bytes properties
• Add multi_stream.vrs test data and comprehensive tests covering imports,
  stream creation, raw record writing, raw bytes reading, and full
  verbatim copy workflow

Differential Revision: D96476344

[meta-codesync]

@danielyan86129 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D96476344.

[meta-cla]

Hi @danielyan86129!

Thank you for your pull request.

We require contributors to sign our Contributor License Agreement, and yours needs attention.

You currently have a record in our system, but the CLA is no longer valid, and will need to be resubmitted.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!