fix: Serialize `bitWidth` field of `FlatBuffers` Time type for `time64` (#190)

## What's Changed

The vendored `ArrowWriterHelper` did not serialize the `bitWidth` field
of the
FlatBuffers `Time` type when writing a `time64` schema. Since the
field's
FlatBuffers default is 32, a `time64` column was encoded as `Time(unit,
32)`,
and servers validating the schema rejected the column with:

```
[UNSUPPORTED_ARROWTYPE] Unsupported arrow type Time(NANOSECOND, 32)
```

This adds `bitWidth: 64` to the `.time64` case in `toFBType`. `time32`
is
unaffected because 32 is the correct default there.

## Testing

Added `TimeBitWidthTests`, which serializes a schema via
`ArrowWriter.toMessage(_:)` and parses the resulting FlatBuffers message
to
assert that `time64` (microseconds/nanoseconds) encodes `bitWidth` 64
and
`time32` (seconds/milliseconds) encodes 32.

The existing round-trip test (`testTimeInMemoryToFromStream`) cannot
catch
this regression because the Swift reader dispatches on `unit` only and
ignores `bitWidth`.