feat: Add support for read pools in postgres

[eidam]

Cloud SQL for PostgreSQL read pools are GA now, adding a support creating a read replica as a read pool.

The google_sql_database_instance resource supports it already, the only difference is node_count and instance_type to be READ_POOL_INSTANCE.

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[imrannayer]

@eidam thx for the PR. Can you plz generate README. Follow this doc

[eidam]

@imrannayer thanks for the review, added node_count validation + updated readme

for the additional read_pool_auto_scale_config i would look into outside of this PR scope later

[imrannayer]

/gcbrun

[imrannayer]

@eidam failing with following error

TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185: Error: Operation failed
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185: 
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185:   on ../../modules/postgresql/variables.tf line 401, in variable "read_replicas":
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185:  401:     condition     = length([for replica in var.read_replicas : false if contains(keys(replica), "node_count") && (replica.node_count < 1 || replica.node_count > 20)]) == 0
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185:     ├────────────────
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185:     │ replica.node_count is null
TestPostgreSqlPscModule 2025-12-04T16:51:27Z command.go:185:

[eidam]

Weird, I was running make lint locally without errors

I likely won’t get into this until eoy, anyone feel free to pick this up

[imrannayer]

fixed in #777

[eidam]

read pool != connection pool, but i will reopen once i get around this again