Support `TimeField`

CrateDB does not support storage for timetz, all times have to be stored either in a `timestamp` or `timestamptz`.

`TimeField` only takes the time part of a timestamp, CrateDB trips if we try to do:

```python
TimeField(datetime.datetime.now().time().isoformat())
# SQLParseException[Cannot cast value `22:43:10.001021` to type `timestamp without time zone`]
```
Because CrateDB needs the full timestmap.

But if we pass a timestamp, the validator of the field trips with:
`django.core.exceptions.ValidationError: ['“2020-01-01T22:43:10.001021” value has an invalid format. It must be in HH:MM[:ss[.uuuuuu]] format.']`

To fix this we should modify the validator of `cratedb.fields.TimeField`

The affected entry point is cratedb.fields.TimeField; start by reading its validator and reproducing the two inputs shown against CrateDB. Done means a timestamp containing a date and time can be accepted for storage while TimeField continues to represent the time portion.

Written by the indexing model from the issue text.