Function
get-trailer-value
Gets the value for the trailer with the given name, or none if the trailer is not present.
If there are multiple values for this header, only one is returned, which may be
any of the values. See get-trailer-values if you need to get all of the values.
This functions returns ok(some(v)) if the trailer with the given name is present,
and ok(none) if no trailer with the given name is present. If max-len is too
small to fit the value, an error.buffer-len error is returned, providing a
recommended buffer size.
get-trailer-value: func(body: borrow<body>, name: string, max-len: u64) -> result<option<list<u8>>, trailer-error>;