Add default roles resource

[rneacsu5]

Closes #38 and #119.

This PR adds a new resource called mysql_default_roles which allows you to set default rolest to users using the ALTER USER ... DEFAULT ROLE syntax.

Example:

resource "mysql_role" "readonly" {
  name = "readonly"
}

resource "mysql_user" "jdoe" {
  user = "jdoe"
  host = "%"
}

resource "mysql_grant" "jdoe" {
  user     = mysql_user.jdoe.user
  host     = mysql_user.jdoe.host
  database = ""
  roles    = [mysql_role.readonly.name]
}

resource "mysql_default_roles" "jdoe" {
  user  = mysql_user.jdoe.user
  host  = mysql_user.jdoe.host
  roles = mysql_grant.jdoe.roles
}

I also added tests for this new resource and created a documentation page.

@petoju What do you think? Could you look over this PR? And thank you for keeping this provider alive!

[petoju]

This was released as a part of provider version 3.0.55.