Provide Theia URI compatible ModelServerClient

[ndoschek]

For Theia related components (e.g. command handlers, diagnostic manager) it would be benficial to be able to inject a TheiaModelServerClient that accepts Theia URIs, which are then internally converted to the URI type of the base ModelServerClient

• package modelserver-theia
  • TheiaModelServerClientApiV2offers the complete API accepting Theia's URI type. Internally converts URI (urijs) and passes on to the ModelServerClientApiV2
• update tests