feat: Add Fields option for rules

[kentquirk]

Which problem is this PR solving?

This adds a new option for Rules -- instead of specifying Field which is a single field name, it's now possible to specify Fields, which takes an array of field names. The evaluator iterates over each field in turn and returns the first field that exists in the incoming span.

This will allow users to write rules that account for field name changes like the upcoming http semantic convention changes.

Short description of the changes

• Add new Fields element to rules data structure
• Validate that both Field and Fields do not exist, both in the loading code as well as the validator
• Modify Init() so that it copies Field to Fields so that the evaluation code is clean
• Modify evaluation code
• Write some tests
• Fix some old tests that didn't call Init()
• Document this in metadata
• Add examples to rules_complete
• Regenerate docs files

fix #952

[MikeGoldsmith]

Looks great 👍🏻

I've left open query regarding fail on first match vs attempting all field names and a suggestion around simplifying the check when copying field into fields.