sdks: breaking changes desired prior to 1.0.0 release

[wkalt]

Here are some breaking changes I think should be made in the go SDK prior to a v1.0.0 release:

• stream handling of attachments: Go: add support for stream handling of attachments #737
• Functional options for writer/lexer: go: switch lexer and writer to functional options pattern #738
• Revision of writer defaults: go writer: default to chunked files #739

Prior to a general v1.0.0 library push, I think we should also make these changes:

• split attachment length out of attachment data field: specification: split attachment length into a separate field from attachment data #736