Backport of Add default intention policy into release/1.18.x

[dhiaayachi]

Backport

This PR is auto-generated from hashicorp#20544 to be assessed for backporting due to the inclusion of the label backport/1.18.

The below text is copied from the body of the original PR.

---

Recommended to review commit-by-commit

Description

Adds a new agent configuration field DefaultIntentionPolicy (default_intention_policy) which controls how service-to-service traffic is authorized in the absence of specific intentions.

DefaultIntentionPolicy can be "allow", "deny", or "", where if left blank it will inherit the default ACL policy.

This field will de-couple the ACL subsystem from intentions, allowing users to incrementally adopt secure configurations one step at a time without dealing with implicit dependencies between the two subsystems.

Testing & Reproduction steps

• Added unit tests which inject default intention policy and observe that it overrides the default ACL policy

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

---

Overview of commits

• 78411b5 - 0da74f7