Allow using Guid and other types for queries

I'm writing an application where I have many related entities and I use Guid most of the time as key type.
However, this causes a lot of issues with queries like the following:

```c#
var userData = userCtx.Query()
.UseKeys(new string[] { userId.ToString() })
.Select(u => new
{
User = u,
Teams = teamCtx.Query().UseKeys(u.Teams.Select(x => x.ToString())).ToList(),
Groups = groupCtx.Query().UseKeys(u.Groups.Select(x => x.ToString())).ToList(),
})
.FirstOrDefault();
```

The generated query is explicitly performing an array mapping with the to-string function:
```sql
SELECT
`Extent1` as `user`,
(SELECT RAW `Extent2` FROM `team` as `Extent2` USE KEYS ARRAY TOSTRING(`Extent3`) FOR `Extent3` IN `Extent1`.`teams` END) as `teams`,
(SELECT RAW `Extent4` FROM `group` as `Extent4` USE KEYS ARRAY TOSTRING(`Extent5`) FOR `Extent5` IN `Extent1`.`groups` END) as `groups`
FROM `user` as `Extent1` USE KEYS ['00000000-0000-0000-0001-000000000001']
LIMIT 1
```
Instead, the query I would expect to get should look like this:

```sql
SELECT
`Extent1` as `user`,
(SELECT RAW `Extent2` FROM `team` as `Extent2` USE KEYS `Extent1`.`teams`) as `teams`,
(SELECT RAW `Extent4` FROM `group` as `Extent4` USE KEYS `Extent1`.`groups`) as `groups`
FROM `user` as `Extent1` USE KEYS ['00000000-0000-0000-0001-000000000001']
LIMIT 1
```

Instead I would like to be able to either directly write
```c#
teamCtx.Query().UseKeys(u.Teams).ToList()
```
or at least be able to use the Linq cast function like this to make it compile properly and generate an efficient query:
```c#
teamCtx.Query().UseKeys(u.Teams.Cast()).ToList()
```

However this last variant is throwing an error: `No coercion operator is defined between types 'System.Guid' and 'System.String'.`

Is there any other way to achieve the query I intended or would this require an adjustment to this library?