Add StreamWriteCapability for further format-based/format-agnostic handling improvements

[cowtowncoder]

(note: similar to #619)

Similar to addition of StreamReadCapability for JsonParser, it seems useful to have indicators that allow addition of some level of format-specific support at databind layer without making it directly aware of details of format backends: to expose set of capabilities that serializers can introspect.

At this point addition is somewhat speculative (i.e. there are areas where it might be handy, but no single specific usage yet), but the idea is to add plumbing in 2.12 so it will be available for later minor versions.