Exposes unsigned long via the fields API.

Unsigned longs default to java signed longs.  That means the upper range
appears negative.  Consumers should use `Long.compareUnsigned(long, long)`
`Long.divideUnsigned(long, long)` and `Long.remainderUnsigned(long, long)`
to correctly work with values known to be unsigned long.

Alternatively, users may treat the unsigned long type as `BigInteger` using
the field API, `field('ul').as(Field.BigInteger).getValue(BigInteger.ZERO)`.
```
field('ul').as(Field.BigInteger).getValue(BigInteger.valueOf(1000))
field('ul').getValue(1000L)
```

This change also implements the beginning of the converters for the fields
API.  The following conversions have been added:
```
ulong <-> BigInteger
long <-> BigInteger
double -> BigInteger
String (parsed as long or double) -> BigInteger
double -> long
String (parsed as long or double) -> long
Date (epoch milliseconds) -> long
Nano Date (epoch nanoseconds) -> long
boolean (1L for true, 0L for false) -> long
```

Fixes: elastic#64361

…/69742-06-ulong-as

…/69742-06-ulong-as