Add server_id

[pasi-paasiala]

Since Topic Index is deprecated (buildingSMART/BCF-API#202), we should add support for server_id also in BCF-XML.

Acceptance criteria

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[ ] server_id is available in BCF-XML
[ ] Index deprecation documentation refers to the server_id property

[jasollien]

Had a comment regarding this: #232 (comment)

[ykulbak]

Closing, duplicate of #232