Update metadata of a Fund.

Note: metadata field that does not appear in the body params will be kept intact. To remove a metadata field, supply it with empty value (""). For example, with the current metadata as {"Oppotunity ID": "ID123", "Client Name": "John Doe"}.
An update with metadata field as {"Partner ID": "ABC-VC", "Client Name": ""} will result in the following metadata {"Oppotunity ID": "ID123", "Partner ID": "ABC-VC"}