ratelimits: combine validation and precomputation into a constructor

Reviewing https://github.com/letsencrypt/boulder/pull/7869/files/b7189f5c31798523fe61d1be0a40363cb9e9fa30..81b616be22710d38b2193f200f63a9c2bed86637#diff-ba4749ac861bae77ee5067820eff8d269f6ede6202108c0238a1809726d066d3R224-R236, I notice that we now have a pattern of partially initializing a struct, then validating its contents, then precomputing some internal fields:

		lim := &limit{
			burst:  v.Burst,
			count:  v.Count,
			period: v.Period,
			name:   name,
		}

		err := validateLimit(lim)
		if err != nil {
			return nil, fmt.Errorf("parsing default limit %q: %w", k, err)
		}

		lim.precompute()

This is a great use case for a constructor that returns an error:

func newLimit(name string, config LimitConfig) (*limit, error) {

That way we can reduce the chance of winding up with a partially-initialized limit object, and also reduce boilerplate a little bit.

Start by reviewing the linked pull request diff and locating limit, validateLimit, precompute, and LimitConfig in the repository. Trace the current initialization call site, then make the constructor own validation and precomputation; done means callers use it without partially initializing limit values.