datastore: refactor Query.Filter() to use separate parameters for field name and operation

[telpirion]

The Query.Filter() method takes a single string parameter that accepts both the field to query for and the operation to perform in the query. The current implementation uses strings.TrimRight() to separate out the field name from the operation, but this solution is problematic as we add more operators.

Source code

Possible solution

Ideally we would refactor Filter() to follow the idiom of Filter(fieldName, operation string, value interface{}), an idiom that is used in other Datastore clients.

Alternatives

Add a new improved Filter() method -- BetterFilter()? -- that uses the better idiom.

Context

This issue is more pronounced because we allow customers to pass in query strings with and without spaces between field names and operators.

[telpirion]

This feature was already added with the FilterField() method ( #6017 ).