feat: support classic-flavored ingest keys

[jharley]

Which problem is this PR solving?

We've now released Ingest Keys, but in order for them to work with Classic environments properly we need to update the key detection logic.

Short description of the changes

• updates the Classic API Key detection logic to understand the shape of a Classic Ingest Key
• makes the previously private Config.isClassic a public method (Config.IsClassic) to allow other projects to use this method and reduce the duplication of logic (i.e. beeline-go)

[kentquirk]

I was slightly worried about performance, and took a minute to run a couple of benchmarks with variations on the regexp.

First, classic keys are hex only, so you can change the classic key regex to a-f instead of a-z. This makes that regex faster.

Second, if you check length manually and then remove the counts from the regexp, it runs literally 1000x faster (7 nS instead of 7 uS!)

	var classicKeyRegex = regexp.MustCompile(`^[a-f0-9]*$`)
	var classicIngestKeyRegex = regexp.MustCompile(`^hc[a-z]ic_[a-z0-9]*$`)

		if len(key) == 0 {
                       return true
		} else if len(key) == 32 {
			return classicKeyRegex.MatchString(key)
		} else if len(key) == 64 {
			return classicIngestKeyRegex.MatchString(key)
		} 
                 return false

[jharley]

Second, if you check length manually and then remove the counts from the regexp, it runs literally 1000x faster (7 nS instead of 7 uS!)

Whoa! Thank you for this: that's huge. Will refactor the shape of the function