Add ToDictionaryAsync and ToHashSetAsync Linq extension methods

[dennis-gr]

Convenience methods to avoid the slightly awkward syntax when doing it manually after calling ToListAsync.

Async-Generator fails to generate tests for ToDictionaryAsync, not sure why it doesn't get recognized.

[fredericDelaporte]

What is awkward? (await query.ToListAsync()).ToHashSet()? Or hashSet.UnionWith(await query.ToListAsync())?

I do not see the need.

[dennis-gr]

What is the awkward? (await query.ToListAsync()).ToHashSet()? Or hashSet.UnionWith(await query.ToListAsync())?

Both: the chained method call with the need to add parantheses around the async call, creating the HashSet yourself also isn't great IMHO.

It's a small usability improvement, I don't see the harm in adding these.

[fredericDelaporte]

I see some harm:

• It may lure users into thinking the thing is optimized in order to directly load the result into a hashset/dictionary instead of going through an intermediate list.
• That is still some additional code to maintain, just for sparing a couple of parenthesis to callers.

Other maintainers may see it otherwise, so, It leave it open.

[dennis-gr]

I see some harm:

* It may lure users into thinking the thing is optimized in order to directly load the result into a hashset/dictionary instead of going through an intermediate list.

Optimizing this would be possible as soon as #2274 is done. I'll add a remark that calling these methods still creates an intermediate list.

* That is still some additional code to maintain, just for sparing a couple of parenthesis to callers.

Like I said, ATM it's a small usability improvement that I think is worth it, with the option to provide a slightly optimized version in the future. EF Core has these methods and it would be nice if NH did too.