feat: add aws_db_proxies data source

[PawelTopors]

Description

Add new aws_db_proxies that will return all the available RDS proxies.

Relations

Closes #41174

References

API: https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DescribeDBProxies.html

Output from Acceptance Testing

make testacc TESTS=TestAccDBProxiesDataSource PKG=rds
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.5 test ./internal/service/rds/... -v -count 1 -parallel 20 -run='TestAccDBProxiesDataSource'  -timeout 360m -vet=off
2025/02/03 13:25:59 Initializing Terraform AWS Provider...
=== RUN   TestAccDBProxiesDataSource_basic
=== PAUSE TestAccDBProxiesDataSource_basic
=== CONT  TestAccDBProxiesDataSource_basic
--- PASS: TestAccDBProxiesDataSource_basic (439.59s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/rds	446.583s

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