Add separate SSH_USER config option

[clarfonthey]

This lets the user shown in SSH URLs be customised even when the built-in server is disabled. In my case, I customised my SSH server to accept logins from a username other than the one that gitea is logged into, and the way I currently do this is by running the built-in SSH server to override the displayed URL without actually using it. I think that this method is much cleaner, and doesn't add too much complexity to the configuration. In fact, IMHO, it actually simplifies things for non-niche users as well.

The value of this will default to the BUILTIN_SSH_SERVER_USER if not provided, which will default to RUN_USER if that's also not provided, meaning that for most people this will "just work." However, I did put some caveats below.

I technically filed a ticket for this a while back but it got closed due to confusion about the actual implementation, so, I don't think it's worth it to link it here. But if anyone is curious, I can probably find it.

⚠️ BREAKING ⚠️

This is technically a breaking change, since people who have modified BUILTIN_SSH_SERVER_USER but don't actually have the built-in server running will have their URLs show the value they chose for that, which will presumably not work. Initially I considered modifying this change to default to BUILTIN_SSH_SERVER_USER only when the built-in server was enabled, and RUN_USER otherwise, but I ultimately chose to abandon this approach since:

1. It's more-complicated in both code and explanation.
2. The number of people who have made these changes is likely small
3. The people who have BUILTIN_SSH_SERVER_USER set to something other than RUN_USER without the built-in server running probably just forgot to remove that section from their config, and probably should fix that
4. Explaining the extra default rules adds more complexity to the ruleset when it makes more sense to just let the variables default in the order of SSH_USER -> BUILTIN_SSH_SERVER_USER -> RUN_USER.

Clarification of my specific use case

This is mostly for the curious people who are wondering how I have this set up, and is not important to the PR itself. Here, I explain how I allow SSH to connect to gitea via a user different than the one gitea uses.

1. I'm running on Arch Linux where gitea is running under a gitea user and group.
2. For the sake of simplicity, let's say that the extra user for SSH is g.
3. I modified SSH_AUTHORIZED_KEYS_COMMAND_TEMPLATE to run a custom script that wraps the gitea serv key command. (Before this option was added, I wrapped the authorized keys command in my SSH config directly to modify the output.)
4. The g user has no permissions, although in the sudoers file it's allowed to run commands as the gitea user without a password and while passing its environment.
5. My wrapper script runs sudo -Eu gitea gitea serv key-$1, where $1 is the passed key ID. It's only allowed to execute as the g user.
6. This user is normally not allowed to connect via SSH, but is allowed to connect via SSH on a separate IP and port only to execute these commands.

[techknowlogick]

One tiny nitpick but otherwise looks good. Thanks for this PR!

[techknowlogick]

Could you match what you have in the markdown docs here %(...

[clarfonthey]

The main reason I didn't was because the style for the docs in both places is inconsistent, and I was mostly copying what I saw. Can still make the change if you think it'll be better that way

[clarfonthey]

cc @techknowlogick since it's been a few days, just wanna ask whether you want me to make the change or not

[techknowlogick]

Apologies for the disappearance.

Could you make these changes? I know that there are inconsistencies between these two files, although hopefully they will be aligned, and to make the effort of that future person less that is why I'm requesting they be aligned.

I don't have super strong feelings about this, and so I will add a LGTM and leave it up to you if you wish to change it so I'm not blocking this PR from being merged :)

[clarfonthey]

Apologies for my disappearance!

Reading this again, I'm not 100% sure what you meant by copying the markdown docs, so, I did what made the most sense to me and put the default directly as the variable value. Also did the same for SSH_BUILTIN_SERVER_USER as well since this directly uses that. Let me know if this is what you'd prefer.

[clarfonthey]

Sorry for taking so long; I rebased this on the latest main and made the docs changes that were requested, at least what I think was requested. Let me know if you have any extra feedback.