fix!: safer_mysql module's assign_public_ip input should be bool type

[leavesster]

Fix

the safer_mysql submodule's assign_public_ip's type is string, but default value is false which is bool type

variable "assign_public_ip" {
  description = "Set to true if the master instance should also have a public IP (less secure)."
  type        = string
  default     = false
}

and this variable is used as bool parameter to ip_configuration.ipv4_enabled

terraform-google-sql-db/modules/safer_mysql/main.tf

Line 54 in 5bcd8a4

ipv4_enabled = var.assign_public_ip

but the ipv4_enabled is require bool type not string.
It also effects example mysql-private at

terraform-google-sql-db/examples/mysql-private/main.tf

Line 97 in 5bcd8a4

assign_public_ip = "true"

current action

if I run terraform apply in example/mysql-private , it will reject to run because of this wrong type.

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[imrannayer]

@leavesster thanks for the PR. lint test is failing. Plz generate docs. You can follow this doc

[imrannayer]

/gcbrun