Add support for copying query builders.

[MrMage]

Creates a copy of the current query builder. Useful for constructing two similar, but not identical, queries.

Example: two queries sorted by different criteria.

let baseQuery = try User.query(on: conn).filter(\.name == "foo")
let sortedByID = baseQuery.copy().sort(\.id, .ascending)
let sortedByName = baseQuery.copy().sort(\.name, .ascending)

Example: two queries, one sorted, one counting. (Used e.g. for pagination; see https://github.com/nodes-vapor/paginator/blob/0c1ce194d3ea4564c782e94ba3ea4f09b5d87ad1/Sources/Paginator/Paginatable/QueryBuilderPaginatable.swift#L55.)

let baseQuery = try User.query(on: conn).filter(\.name == "foo")
let sortedQuery = baseQuery.copy().sort(\.id, .ascending)
let countQuery = baseQuery.copy().count()

(Without the copy, this will add the ORDER BY clause to the query, which Postgres chokes on.)

[MrMage]

@tanner0101 would you mind taking a look at this? (CI is broken by an outdated Swift version, unrelated to this PR.)

[tanner0101]

LGTM, thanks @MrMage :)

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