Matching an element in a list

There are several use cases in Envoy when we need to iterate over a list of items and match each item individually:
* selecting the first ALPN for which a nested filter chain matcher accepts;
* matching an item in DNS SAN list in a TLS cert;
* matching an IP value in XFF header.

Ideally, we want to support all existing matchers for these use cases, instead of adapting all matchers to expect a list.

How should we define this in xds?

Option 1: Add a common input that takes another input and produces an iterator of data items. This requires some changes in the implementations to return an iterator for data inputs.
Option 2: Add a custom matcher that takes another matcher as input. The nested matcher can be any of string, custom input, prefix map, exact map, or custom sub-linear matcher. The custom matcher parses the input and then attempts to match elements one by one.

Ref: https://github.com/envoyproxy/envoy/issues/20666