(appsync): support filters in DynamoDB Query

Currently, the Query mapping template doesn't support filters.

### Use Case

Query DynamoDB with additional filters.

### Proposed Solution

Provide an optional way to specify filter conditions like the already existing key conditions.

### Other

Here is an example of what is not currently possible. It is taken directly from the examples inside the appsync was console.

```
## Below example shows an example of a query against a table, indexed by the "name" attribute
## The query returns any items in the index matching a certain "name", and then applies a
## filter to the results to match only the results with a certain "city" attribute
## "expressionNames" is used as a placeholder or alias to the DynamoDB attribute, and is
## necessary for certain reserved attribute keywords like "name" or "owner"
## "filter" : "expression" is the logical condition applied after the query runs, you can use:
## - NOT_NULL(), NULL(), CONTAINS(), NOT_CONTAINS()
## - This also works for attributes that are sets. For example, you can use "expression" : "contains(projects, :favfood)
## to check an attribute called "projects" of type String Set ("SS") for matches of a certain favorite food
## - BEGINS_WITH(), IN(), BETWEEN()
## - Comparators: >, <. >=, <= can be used for the "expression" key inside a filter
## - For example, "expression" : "#age > :age" would work on an attribute of type number
## Read more: https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-filter

{
"version" : "2017-02-28",
"operation" : "Query",
"index" : "name-index",
"query" : {
"expression": "#name = :name",
"expressionNames" : {
"#name" : "name"
},
"expressionValues" : {
":name" : {
"S" : $util.dynamodb.toDynamoDBJson($ctx.args.name)
}
}
},
"filter" : {
"expression" : "contains(#city, :city)",
"expressionNames" : {
"#city" : "city"
},
"expressionValues" : {
":city" : $util.dynamodb.toDynamoDBJson($ctx.args.city)
}
}
}
```

* [x] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

Start with the AppSync DynamoDB Query mapping template and the AWS filter documentation linked in the issue. Compare the requested filter block with the existing key-condition handling, then verify that Query mappings can accept an optional filter with the documented expressions and values.

Written by the indexing model from the issue text.