Add custom JsonConverter for BinaryData that converts to Base64 string

[ovidiucosteanet]

• Created custom converter as per described in the issue
• Added the converter also to the ref BinaryData class
• Added appropriate tests for serialization and deserialization of a class that has a member of BinaryData type

Fixes #70267

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Issue Details

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• Created custom converter as per described in the issue
• Added the converter also to the ref BinaryData class
• Added appropriate tests for serialization and deserialization of a class that has a member of BinaryData type

Fixes #70267

Author:	ovidiucosteanet
Assignees:	-
Labels:	area-System.Memory, new-api-needs-documentation
Milestone:	-

[ovidiucosteanet]

Thank you @adamsitnik!