Ability to reference custom logic in Hasura's generated `BoolExp`s

### Is your proposal related to a problem?

Currently, if a user would like to expose features such as full text search or fuzzy search in the Hasura Graphql schema, they must create a postgres function and track it within Hasura. This works up to a point, but it doesn't play nicely with Hasura's generated `BoolExp`s.

For example, say the client wishes to perform a fuzzy search on post body content. The client might perform the following pseudo graphql query:

```gql
query MySearchQuery($bodyContentQuery: String!) {
searchThreads(
args: { search: $bodyContentQuery }
) {
...fields
}
}
```

Well what if the client wishes to search for posts that contain post body content OR which were sentAt a specific time? This can't easily be done because the `$bodyContentQuery` value cannot be used in the `_or` section of a `where` BoolExp.

### Describe the solution you'd like

I desire to do something like the following:

```gql
query MySearchQuery($bodyContentQuery: String!, $sentAtTime: String!) {
searchThreads(
where: {
_or: [
{ body: { _fuzzysearch: $bodyContentQuery } },
{ sentAt: { _eq: $sentAtTime } },
]
}
) {
...fields
}
}
```

Alternatively, it would also be acceptable if I could reference [table computed fields](https://hasura.io/docs/latest/schema/postgres/computed-fields/#2-table-computed-fields) in `BoolExp`. For example

```gql
query MySearchQuery($bodyContentQuery: String!, $sentAtTime: String!) {
threads(
where: {
_or: [
{ searchThreads: { args: $bodyContentQuery } },
{ sentAt: { _eq: $sentAtTime } },
]
}
) {
...fields
}
}
```

The second approach seems more flexible than the first.

### Describe alternatives you've considered

The only alternative I'm aware of is to not use Hasura's generated `BoolExp` (i.e. not use the `where` option) and instead build equivalent custom functionality in a postgres function. This seems very time consuming.

I initially got excited thinking that [Table Computed Fields](https://hasura.io/docs/latest/schema/postgres/computed-fields/#2-table-computed-fields) might solve this problem, but they don't appear to be added as options to the relevant `BoolExp`.

Start by examining how Hasura's generated GraphQL BoolExp inputs are built and how PostgreSQL functions and table computed fields are represented. Compare the requested custom logic and computed-field examples, then define completion as allowing that logic to participate in BoolExp _or conditions while preserving the existing sentAt filter behavior.

Written by the indexing model from the issue text.