Use Go validator framework to succinctly validate requests

[brandur]

A general problem with Go is that validating things is a very noisy
affair involving long lists of if statements combined with error returns
for every field on a struct.

A technique that we've been using for a while is to use the Go validator
framework [1] that allows fields to be tagged with succinct validation
syntax for a variety of different things. e.g.

type jobCancelRequest struct {
    JobIDs []int64String `json:"ids" validate:"required,min=1,max=1000"`
}

I'm not sure the use of something like this is necessary for a project
that's a dependency like core River itself, but but internal use on more
of an "application" project like River UI, it might be helpful.

We combine the validator with the new API framework from #63 so that
incoming request structs are validated automatically for every endpoint,
which shaves a lot of lines of otherwise necessary validation code out
of individual API endpoint definitions.

[1] https://github.com/go-playground/validator?tab=readme-ov-file

[brandur]

@bgentry This is a pretty good option for getting some better request validation going on. Thoughts?

[bgentry]

Looks solid, excited to try this out!

[bgentry]

do you prefer this style with fallthrough over putting all the values in a single case clause?

[brandur]

I find it nominally easier to read because it's less cramped, but no strong opinion either way. The one line with commas is probably more conventional, so I changed it to that.

[brandur]

ty.