[Appender] Cast float64 to numeric destination type

[taniabogatsch]

Depends on #196.

Adds an implicit cast from the source type float64 to any expected numeric destination (column) type. These changes, and the previous work in #196, enable the appender to append generic interface inputs, also those retrieved from json.Unmarshal.

https://go.dev/blog/json

The json package uses map[string]interface{} and []interface{} values to store arbitrary JSON objects and arrays; it will happily unmarshal any valid JSON blob into a plain interface{} value. The default concrete Go types are:
bool for JSON booleans,
float64 for JSON numbers,
string for JSON strings, and
nil for JSON null.