HEAD requests are not available

Description

Not sure if this is a bug or working as intended, but I have noticed that TesseraCT (at least the POSIX one) does not implement HEAD requests, only POST and GET:

Compare:

$ curl -Ssv  https://halloumi2025h2.log.ct.ipng.ch/ct/v1/get-roots -o /dev/null
...
< HTTP/2 200 
< server: nginx/1.26.3
< date: Wed, 27 Aug 2025 21:03:22 GMT
< content-type: text/plain; charset=utf-8
< vary: Accept-Encoding
< x-ipng-frontend: nginx0-chrma0
< 
{ [8060 bytes data]

With:

$ curl -ISsv  https://halloumi2025h2.log.ct.ipng.ch/ct/v1/get-roots -o /dev/null
< HTTP/2 405 
< server: nginx/1.26.3
< date: Wed, 27 Aug 2025 21:04:10 GMT
< content-type: text/plain; charset=utf-8
< content-length: 44
< x-content-type-options: nosniff
< x-ipng-frontend: nginx0-chplo0
< 
{ [0 bytes data]

This is not a frontend issue, hitting the IPv4 / IPv6 endpoint directly yields the same result:

ctlog@ctlog1:~$ curl -I  localhost:16900/ct/v1/get-roots 
HTTP/1.1 405 Method Not Allowed
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Wed, 27 Aug 2025 21:05:52 GMT
Content-Length: 44

ctlog@ctlog1:~$ ps auxw | grep 16900
ctlog      51635  2.3  0.2 4418792 45844 ?       Ssl  Aug26  52:54 /home/ctlog/bin/tesseract-posix --private_key=/ssd-vol0/enc/tesseract/keys/lipase2025h2.pem --origin=lipase2025h2.log.ct.ipng.ch --storage_dir=/ssd-vol0/logs/lipase2025h2/data --roots_pem_file=/ssd-vol0/logs/lipase2025h2/data/roots.pem --http_endpoint=[::]:16900 --not_after_start=2025-07-01T00:00:00Z --not_after_limit=2026-01-01T00:00:00Z

Perhaps we can consider documenting that HEAD requests are ineffective in TesseraCT, or update it to implement HEAD requests.

Start at the HTTP handling for the /ct/v1/get-roots entry point and compare its current GET and POST method behavior with the reported HEAD request. Decide whether HEAD should be supported or documented as ineffective; done when the chosen behavior is covered by the relevant checks or documentation.