Support external sharing automations

[meiersi-da]

• implement as outlined below
• adjust docs

canton.validator-apps.validator_backend {
  reward-sharing-config-by-party = {
    # 30% -> charlie, 20% -> dave, remaining 50% stays with alice
    "alice::1220abc...def" = {
      // There is external sharing automation running for this party,
      // do not mint unshared coupons and do not run the built-in sharing automation for this party.
      sharing-automation = "external"
    }

    # External party bob sends 100% of rewards to a treasury party
    "bob::1220bbb...ccc" = {
      sharing-automation = "built-in"  // does not need to be set, but is shown here for clarity
      min-ttl-after-sharing = 25h
      # Max coupons to share per trigger run (default: 100)
      batch-size = 50
      beneficiaries = [
        { beneficiary = "treasury::1220eee...fff", percentage = 1.0 },
      ]
    }
  }
}