feature: support separate annotation files

[phanak-sap]

It possible, that not everything is in $metadata file. Sometimes, additional annotations come as separate request to different, unknown URL, (e.g. SAP fiori app in RAP framework).

refer to odata-library:

• annotationsUrl
• https://github.com/SAP/odata-library/blob/master/doc/INITIALIZE.md

Example:

• Fiori app Profit Center Changelog.
• URL: /IWFND/CATALOGSERVICE;v=2/Annotations(TechnicalName='FAC_PROFIT_CENTER_CHANGLOG_VAN',Version='0001')/$value/?sap-language=EN&sap-client=715";

[jfilak]

Good idea. Do that.

[phanak-sap]

Notes:

• There may be multiple external annotations URLs, odata-library just expects one, since that is the SAP systems default.

• This seems on first glance that is logical that V2 odata service will have V2 external annotations (which is checked in pyodata during metadata parsing), but actually V2 services for fiori apps have V4 external annotations, just that they are backward compatible in this instance, they are applied to V2 metadata; but current pyodata logic would not recognize V4 xml declarations as valid.

• This enhancement should actually be considered a subtask for V4 support.