date in flag inside vulnerabilities and title inside note data type made optional

[S-Panta]

Part of #4

title should also be optional in note data type of document .Ref:https://docs.oasis-open.org/csaf/csaf/v2.0/os/csaf-v2.0-os.html#3217-document-property---notes

Value type of every such Note item is object with the mandatory properties category and text providing a place to put all manner of text blobs related to the current context. A Note object MAY provide the optional properties audience and title.

In CSAF, date property in flag inside vulnerabilities should be optional and some URL may not contain this.

`In addition, any Flag item MAY provide the three optional properties Date (date), Group IDs (group_ids) and Product IDs (product_ids).

https://docs.oasis-open.org/csaf/csaf/v2.0/os/csaf-v2.0-os.html#3235-vulnerabilities-property---flags

Also, product id should be in array and not be single entity.

Product IDs (product_ids) are of value type Products (products_t) and contain a list of Products the current flag item applies to.

[Salipa-Gurung]

csaf/csaf/parser.py

Line 179 in 271e07a

vuln_info.set_value(reference["category"], [reference.get("summary",""), reference.get("url","")])

I guess above reference["category"] also needs to be changed to "category"