Add `etcd` as input and cache.

[danriedl]

This PR adds:

• feat: add etcd as cache resource. #1154
• feat: add etcd as input to watch a key. #1154

Following configs can be used in parallel to test this PR:

watch-key.etcd.yaml:

input:
  etcd:
    endpoints:
      - localhost:2379
    key: "test" 
pipeline:
  processors: []
output:
  stdout:
    codec: lines

write-key.etcd.yaml:

input:
  generate:
    mapping: |
      root = {
        "id": nanoid(5, "ABCDEFGHIJKL")
      }
    interval: 1000ms
pipeline:
  processors: []
output:
  cache:
    target: etcd
    key: test
cache_resources:
  - label: etcd
    etcd:
      endpoints:
        - localhost:2379