feat: Allow URL encoded dataset in libhoney endpoint paths

[MikeGoldsmith]

Which problem is this PR solving?

Allow URL encoded dataset names in /event and /batch endpoint.

For example, to send data to a dataset with the name foo/bar, you can now send data like this:

curl -i -H "x-honeycomb-team: <api-key>" -X POST 'http://localhost:8080/1/events/foo%2Fbar' -d '{"name":"mike"}'

The dataset name foo%2Fbar is URL decoded into foo/bar and used as the dataset name. When forwarding events to the Honeycomb API, the dataset name is URL encoded again to ensure it's received into HNY correctly.

Short description of the changes

• Update mux router to allow encoded paths
• Add utility function to get the dataset from mux vars and url escape the value
• Add tests for utility function
• Simplify batch event processing by removing batchedEventToEvent, adding getSampleRate batchEvent member func and re-using apiHost and dataset

[MikeGoldsmith]

Waiting on the following PR:

• maint: Bump libhoney-go to v1.23.1 #1200

[kentquirk]

I don't need to block it, but given that there's an old TODO there and this code is being touched...