add warning upon successful md5 password auth

From a related discussion last year [0]:

On Tue, Jun 03, 2025 at 12:09:50PM -0500, Nathan Bossart wrote:
> On Tue, Jun 03, 2025 at 09:43:59AM -0500, Nathan Bossart wrote:
>> On Tue, Jun 03, 2025 at 10:34:06AM -0400, Tom Lane wrote:
>>> If we really want to be in peoples' face about this, the thing
>>> to do is to print a warning every time they log in with an MD5
>>> password.  Also, to Michael's point, that really would be exactly
>>> the same place where the eventual "sorry, not supported anymore"
>>> message will be.
>> 
>> I held off on this because I was worried it might be far too noisy.  That
>> does seem like it has the best chance of getting folks' attention, though.
>> If it's too noisy, users can always turn off the warnings.
> 
> Here is a draft-grade patch that adds a WARNING upon successful
> authentication with an MD5 password.  It's a little hacky because AFAICT we
> need to wait until well after authentication (for GUCs to be set up, etc.)
> before we actually emit the WARNING.  When the time comes to remove MD5
> password support completely, we'll need to do something like modify
> CheckMD5Auth() to always return STATUS_ERROR with an appropriate logdetail
> message.

Since I just added a "connection warnings" infrastructure in commit
1d92e0c2cc, I thought it might be a good time to revisit this idea.
Attached is an updated patch.  I'm not sure this is v19 material.  It could
make sense to wait until v20 or something.  But I figured it was worth at
least having the discussion.

[0] https://postgr.es/m/aD8sXgfJeIGLc7-t%40nathan

-- 
nathan