[Agent] Inject index with sane defaults to configuration

[michalpristas]

Build on top of: #15809

Added one extra rule for injecting index into inputs based on provided type and detected namespace/dataset type.

Other option was to use decorator but at the time of decorating there are already some pieces of config missing.

Draft until final configuration format is settled and mentioned PR is in, but READY for review.

It overwrites index specified in input.

Fixes: #15690
Fixes: #15691

[elasticmachine]

Pinging @elastic/ingest (Project:fleet)

[ph]

LGTM, I am going to approve this. the final format will be a bit different see https://gist.github.com/ph/c7a83c8dee58a90051ceb6b0c2cbad5b not final and not ready for implementation

[michalpristas]

created followups: #16210 #16209

[michalpristas]

Tested with this config. careful monitoring moved to settings.monitoring

outputs:
  default:
    type: elasticsearch
    api_key: VuaCfGcBCdbkQm-e5aOx:ui2lp2axTNmsyakw9tvNnw
    hosts: ["localhost:9200"]
    ca_sha256: "7lHLiyp4J8m9kw38SJ7SURJP4bXRZv/BNxyyXkCcE/M="
  monitoring:
    type: elasticsearch
    api_key: VuaCfGcBCdbkQm-e5aOx:ui2lp2axTNmsyakw9tvNnw
    hosts: ["localhost:9200"]
    ca_sha256: "7lHLiyp4J8m9kw38SJ7SURJP4bXRZv/BNxyyXkCcE/M="

datasources:
  - use_output: default
    inputs:
    - type: logs
      streams:
        - dataset: nginx.acccess
          paths: /var/log/nginx/access.log
    - type: metrics/nginx
      streams:
        - dataset: nginx.stub_status
          metricset: stub_status

settings.monitoring:
  enabled: true
  use_output: monitoring
  logs: true

management:
  host: "localhost"

config:
  reload: 123

download:
  sourceURI: "https://artifacts.elastic.co/downloads/beats/"
  target_directory: "/{user_home}/elastic/downloads"
  timeout: 30s
  pgpfile: "/{user_home}/elastic/elastic.pgp"
  install_path: "/{user_home}/elastic/install"

process:
  min_port: 10000
  max_port: 30000
  spawn_timeout: 30s

retry:
  enabled: true
  retries_count: 3
  delay: 30s
  max_delay: 5m
  exponential: false