Root service payload builder

[berezovskyi]

Would be good to have support for constructing Root service payloads (which have to be constructed as strings due to non-conformant Jazz RDF/XML parser expectations on that document structure).

Responsibilities:

• set up all the mandatory props (XML encoding for the values despite being mostly interpolation-based)
• set up optional domains (duplicating old and new declarations for compat)
• oauth1.0a declaration (+provisional sub-declaration optionally although it's de-facto mandatory for Jazz)