Remove AsyncClientConnect and AsyncSecureClientConnect in favor of async constructors

[ErwanDL]

This is the second PR associated with #1532, removing the intermediate AsyncClientConnect object in favor of async functions to construct the AsyncClient, as discussed on the issue.

On another note, I am quite confused by the AsyncClient vs AsyncClientConnect distinction. Are there reasons not to define AsyncClient::new and AsyncClient::connect as async fn that return an AsyncClient once they have been awaited ? This could remove the need for this intermediary AsyncConnect object.

I think those functions can be simplified. The only reason I can think that didn't happen is because AsyncClient is older than async/await in Rust. Most likely it can be simplified to be async fn instead of Future implementations. If that's something you'd be interested in working on, it would be appreciated...

I also removed AsyncSecureClientConnect and make similar changes to AsyncDnssecClient as they were also based on AsyncClientConnect/AsyncClient.

There should be very little to no changes in usage for users, and the tests still all pass on my machine without having had to make any change to them.

[bluejekyll]

Thank you for these changes. I'll try to find some time tomorrow to review, but it might be another day or two after that.

[codecov]

Codecov Report

Merging #1541 (a953a52) into main (69829fa) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1541      +/-   ##
==========================================
- Coverage   83.41%   83.37%   -0.04%     
==========================================
  Files         170      171       +1     
  Lines       16848    16936      +88     
==========================================
+ Hits        14053    14120      +67     
- Misses       2795     2816      +21     

[bluejekyll]

This looks great. I'm surprised no changes are necessary to tests, but that's a good thing. Means the API is pretty consistent.

Thank you for cleaning this up!