DBAAS-3906: Include updatePool for DB Clusters

[DMW2151]

Adds updatePool + updatesDatabaseService, allows for Postgres pool updates through godo, makes a PUT request to /v2/databases/$uuid/pools/$poolname.

import (
    "context"
    "github.com/digitalocean/godo"
)

func main() {

    pat := "..."
    dbUUID := "..."
    client := godo.NewFromToken(pat)
    
    _, err := client.Databases.UpdatePool(context.Background(), dbUUID, "backend-pool", &godo.DatabaseUpdatePoolRequest{
        User:     "username",
        Size:     14,
        Database: "defaultdb",
        Mode:     "transaction",
    })
}

[ChiefMateStarbuck]

great work

[ChiefMateStarbuck]

are any of the struct vales within the update request required?

[DMW2151]

Yes, size, database, and mode.

[ChiefMateStarbuck]

one nit