util/httpm: add new package for prettier HTTP method constants

See package doc.

Change-Id: Ibbfc8e1f98294217c56f3a9452bd93ffa3103572
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f