feat: add ability to configure Recursion Desired (RD) bit for DNS read queries only

[dmachard]

This PR introduces a new configuration option: recursive (Boolean), which allows users to control whether the Recursion Desired (RD) bit is set for DNS read (resolution) queries.

Purpose

In our current architecture, DNS update requests are sent to an authoritative DNS server, while resolution (read) queries are sent to a recursive resolver. However, without this change, read queries are rejected by our recursive resolver because the Recursion Desired (RD) flag is set to 0 by default. Recursive resolvers expect this flag to be explicitly set to 1 in order to process and resolve the query.

By introducing the recursive configuration option, we allow users to control the RD bit for read queries only, ensuring compatibility with recursive resolvers and avoiding rejected requests.

This change increases flexibility for mixed DNS infrastructures where update and resolution paths differ.

New setting

The following configuration field has been added under the update block:

• recursive (Boolean): Whether DNS resolution queries should be made recursively. When set to true, the DNS server is expected to perform full resolution and return a complete answer. When set to false, the server may return a referral to other name servers. Defaults to false. This value can also be sourced from the DNS_UPDATE_RECURSIVE environment variable.

Additional Notes

• This setting only affects read queries, not dynamic updates.
• No changes were made to the provider plugin behavior or logic unrelated to this flag.
• Documentation has been updated accordingly.
• Unit tests have been added to cover both recursive and non-recursive query behavior.

[hashicorp-cla-app]

All committers have signed the CLA.

[dmachard]

@bbasata Any chance to get some feedback or to approve the tests running on this PR?