feat: Allow additional login params, Introduce LogInValidator

[imorland]

Precursor to https://discuss.flarum.org/d/31904-enabling-cloudflare-turnstile-on-login

Changes proposed in this pull request:

• Extract params into an extendable function so that additional params can be added in LogInModal
• LogInController runs validation of the supplied params against a new LogInValidator - empty ruleset by default.

Reviewers should focus on:

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[SychO9]

Hmm, an event doesn't really seem appropriate to me. I'd rather we create a login validator, empty by default, and extensions can extend the validator instead.

[imorland]

🤔 yeah... That would make more sense, actually. On it...

[davwheat]

Why not both?

That would allow for processing of data from a user logging in, and the more traditional approach to validation.

[SychO9]

I don't know, it's not apparent to me what the processing of data could be, and if whatever it is it couldn't be handled using the exsiting extension API (like we did here with the validator), so I prefer dealing with one use case at a time to avoid over-engineering it.