dnsdist: Grant unidirectional HTTP/3 streams for DoH3

[rgacogne]

Short description

While unidirectional streams are not needed for DNS over QUIC, they are required by the HTTP/3 RFC and thus needed for DNS over HTTP/3. This change makes curl and Firefix happy with dnsdist's DoH3 implementation.

Many thanks to hawk on IRC for bringing it up!

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 7450026464

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 57.69%

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Totals
Change from base Build 7448235306:	0.009%
Covered Lines:	107732
Relevant Lines:	155397

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💛 - Coveralls

[hlindqvist]

Excellent, this does indeed seem to fix the problem!

[rgacogne]

Thanks for testing and reporting back!