Improve IAsyncEnumerable usage

[viceroypenguin]

This PR does two things:

• Updates InsertWithOutputAsync, UpdateWithOutputAsync, and DeleteWithOutputAsync to return IAsyncEnumerable properly. This is a binary-compatible change, as the existing methods are kept. However, these methods are now marked with [Obsolete] to encourage source migration to the newer IAsyncEnumerable method.
• Adds QueryToAsyncEnumerable in places where QueryToArrayAsync and QueryToListAsync are provided. This allows consumers to use IAsyncEnumerable if desired. No obsoletions are planned here, though consumers are allowed and encouraged to upgrade if the query has many rows.

💣 Breaking Change

• The new versions of XxxWithOutputAsync are not source compatible, so this is a v6.0 breaking change. Consumers will need to add .ToListAsync() or similar to address this change. This is to restore consistency between InsertWithOutputAsync/UpdateWithOutputAsync/DeleteWithOutputAsync with MergeWithOutputAsync; this consistency has been chosen to use the more flexible and modern approach.

Replaces #4323

[viceroypenguin]

/azp run test-all

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Shane32]

Just for clarity when reviewing this PR, I'd like to point out the difference between the old and new signatures:

// old
public static Task<TOutput[]> DeleteWithOutputAsync<TSource,TOutput>(
    this IQueryable<TSource>           source,
    Expression<Func<TSource, TOutput>> outputExpression,
    CancellationToken                  token = default)

// new methods
public static IAsyncEnumerable<TOutput> DeleteWithOutputAsync<TSource,TOutput>(
    this IQueryable<TSource>           source,
    Expression<Func<TSource, TOutput>> outputExpression)

public static Task<TOutput[]> DeleteWithOutputAsync<TSource, TOutput>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TOutput>> outputExpression,
    CancellationToken token)

This means that the following code will immediately break compilation for v6 due to the cancellation token not being specified:

// old syntax; immediately broken with v6
var ret = await query.DeleteWithOutputAsync(x => x.Id);

// required fix to use obsolete method
var ret = await query.DeleteWithOutputAsync(x => x.Id, default);

As specified, however, the change is binary compatible - already-compiled code will continue to function with v6, as the optional parameter is built into the caller's compiled code and so will match on the obsolete method.

UPDATE: this is inaccurate because this was removed from the obsolete method. Users will be required to rewrite usage of these methods with the new syntax.

[viceroypenguin]

/azp run test-all

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[linq2dbot]

Test baselines changed by this PR. Don't forget to merge/close baselines PR after this pr merged/closed.

[sdanyliv]

@viceroypenguin , check mergeability with this PR #3401

[viceroypenguin]

@viceroypenguin , check mergeability with this PR #3401

@sdanyliv no merge conflicts; and shouldn't have any impact on #3401.

[MaceWindu]

Why we have this overload for linq2db-mapped method?

[MaceWindu]

and why for template-based specification it is only 2 overloads?

[MaceWindu]

minor updates needed