allow defaults top option to use to set defaults in yaml using anchors

[evaldasc]

I believe it would be useful to have an area to set defaults inside yaml configuration file, using anchors.
Those can be reused below, making configuration less redundant.

it is currently possible to use anchors in the configs section - making first configuration id to be a template to others - which is fine, but option with defaults would be easier to read.

[warber]

Hi @evaldasc, can you give an example?

[evaldasc]

certainly, @warber

lets take this storage settings config applied to 4 hosts groups with anchors in the first one:

configs:
- id: log-storage-settings-kong-non-a
  config: &config
    template: kong.json
    skip: false
  type:
    settings:
      <<: &settings
        schema: builtin:logmonitoring.log-storage-settings
        schemaVersion: 1.0.7
      scope:
        type: environment
        name: EnonAkongRappTa
  groupOverrides: &overrides
  - group: non
    override:
      skip: true
- id: log-storage-settings-kong-prod-a
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EprodAkongRappTa
  groupOverrides: *overrides
- id: log-storage-settings-kong-non-b
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EnonAkongRappTb
  groupOverrides: *overrides
- id: log-storage-settings-kong-prod-b
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EprodAkongRappTb
  groupOverrides: *overrides

we could have defaults moved separately so all configs are "equal", like this:

defaults:
  config: &config
    template: kong.json
    skip: false
  settings: &settings
    schema: builtin:logmonitoring.log-storage-settings
    schemaVersion: 1.0.7
  groupOverrides: &overrides
  - group: non
    override:
      skip: true
configs:
- id: log-storage-settings-kong-non-a
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EnonAkongRappTa
  groupOverrides: *overrides
- id: log-storage-settings-kong-prod-a
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EprodAkongRappTa
  groupOverrides: *overrides
- id: log-storage-settings-kong-non-b
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EnonAkongRappTb
  groupOverrides: *overrides
- id: log-storage-settings-kong-prod-b
  config: *config
  type:
    settings:
      <<: *settings
      scope:
        type: environment
        name: EprodAkongRappTb
  groupOverrides: *overrides

let me know if example is enough