Add usePrivateIP for cloudsql postgresql instances

[KyriosGN0]

🛠️ Description

add the ability to speificy if the cloudsql postgresql instance has private ip we should use
right now we fail if the instance has only public ip

Closes #26827

[sviatlo]

Hello @hsimon-hashicorp!
We've been dreaming of using Vault to authenticate with gcp sql. Is there any chance you can help to facilitate/expedite the review of this PR?
Thank you in advance!

[heatherezell]

Hello @hsimon-hashicorp! We've been dreaming of using Vault to authenticate with gcp sql. Is there any chance you can help to facilitate/expedite the review of this PR? Thank you in advance!

Hi there! Thank you for the ping (genuinely, I appreciate it!) as well as your enthusiasm for this PR. I will check with our engineering and product management teams to see if we can get some eyes on this. I can't make any guarantees about timelines, but this does give me more data about community sentiment which is very valuable to me. Thanks again! ^_^

[KyriosGN0]

Hey @hsimon-hashicorp
Is there possibly some update about my PR?

[robmonte]

Hi @KyriosGN0
Thanks for the contribution!

In addition to the nit I left in the code, two additions are needed.

1. A changelog file needs to be added.
   A new file named 26828.txt should be placed in the changelog directory with the following contents:

```release-note:improvement
secrets/database: Add support for GCP CloudSQL private IP's.
```

2. Documentation updates are needed.
   The "Configure connection" section of the API docs located here will need to be updated to include this new configuration field. That file is located at website/content/api-docs/secret/databases/postgresql.mdx
   
   The Setup portion of the "Authenticating to Cloud DBs via IAM" section of the feature documentation located here will need to be updated to include its use. This can be as simple as adding a new line to both CLI command examples to say use_private_ip="false". That file is located at website/content/docs/secrets/databases/postgresql.mdx.

[KyriosGN0]

hey @robmonte, Thanks you for the review, i addressed the nit in the code and added the required docs changes

[KyriosGN0]

@robmonte could you trigger the tests again ?