Add new connection strategies and mark strategy as required

New values (`apple`, `oidc`, `line`) are from the documentation:
https://auth0.com/docs/api/management/v2#!/Connections/post_connections

Additionally, `strategy` has been marked as required as the API states
it is a mandatory parameter.

auth0/resource_auth0_connection.go

@@ -35,22 +35,22 @@ func newConnection() *schema.Resource {
 			},
 			"strategy": {
 				Type:     schema.TypeString,
-				Optional: true,
+				Required: true,
 				ValidateFunc: validation.StringInSlice([]string{
-					"ad", "adfs", "amazon", "dropbox", "bitbucket", "aol",
+					"ad", "adfs", "amazon", "apple", "dropbox", "bitbucket", "aol",
 					"auth0-adldap", "auth0-oidc", "auth0", "baidu", "bitly",
 					"box", "custom", "daccount", "dwolla", "email",
 					"evernote-sandbox", "evernote", "exact", "facebook",
 					"fitbit", "flickr", "github", "google-apps",
 					"google-oauth2", "guardian", "instagram", "ip", "linkedin",
-					"miicard", "oauth1", "oauth2", "office365", "paypal",
+					"miicard", "oauth1", "oauth2", "office365", "oidc", "paypal",
 					"paypal-sandbox", "pingfederate", "planningcenter",
 					"renren", "salesforce-community", "salesforce-sandbox",
 					"salesforce", "samlp", "sharepoint", "shopify", "sms",
 					"soundcloud", "thecity-sandbox", "thecity",
 					"thirtysevensignals", "twitter", "untappd", "vkontakte",
 					"waad", "weibo", "windowslive", "wordpress", "yahoo",
-					"yammer", "yandex",
+					"yammer", "yandex", "line",
 				}, true),
 				ForceNew: true,
 			},

website/docs/r/connection.html.md

@@ -70,7 +70,7 @@ Arguments accepted by this resource include:
 
 * `name` - (Required) String. Name of the connection.
 * `is_domain_connection` - (Optional) Boolean. Indicates whether or not the connection is domain level.
-* `strategy` - (Optional) String. Type of the connection, which indicates the identity provider. Options include `ad`, `adfs`, `amazon`, `aol`, `auth0`, `auth0-adldap`, `auth0-oidc`, `baidu`, `bitbucket`, `bitly`, `box`, `custom`, `daccount`, `dropbox`, `dwolla`, `email`, `evernote`, `evernote-sandbox`, `exact`, `facebook`, `fitbit`, `flickr`, `github`, `google-apps`, `google-oauth2`, `guardian`, `instagram`, `ip`, `linkedin`, `miicard`, `oauth1`, `oauth2`, `office365`, `paypal`, `paypal-sandbox`, `pingfederate`, `planningcenter`, `renren`, `salesforce`, `salesforce-community`, `salesforce-sandbox` `samlp`, `sharepoint`, `shopify`, `sms`, `soundcloud`, `thecity`, `thecity-sandbox`, `thirtysevensignals`, `twitter`, `untappd`, `vkontakte`, `waad`, `weibo`, `windowslive`, `wordpress`, `yahoo`, `yammer`, `yandex`.
+* `strategy` - (Required) String. Type of the connection, which indicates the identity provider. Options include `ad`, `adfs`, `amazon`, `aol`, `apple`, `auth0`, `auth0-adldap`, `auth0-oidc`, `baidu`, `bitbucket`, `bitly`, `box`, `custom`, `daccount`, `dropbox`, `dwolla`, `email`, `evernote`, `evernote-sandbox`, `exact`, `facebook`, `fitbit`, `flickr`, `github`, `google-apps`, `google-oauth2`, `guardian`, `instagram`, `ip`, `line`, `linkedin`, `miicard`, `oauth1`, `oauth2`, `office365`, `oidc`, `paypal`, `paypal-sandbox`, `pingfederate`, `planningcenter`, `renren`, `salesforce`, `salesforce-community`, `salesforce-sandbox` `samlp`, `sharepoint`, `shopify`, `sms`, `soundcloud`, `thecity`, `thecity-sandbox`, `thirtysevensignals`, `twitter`, `untappd`, `vkontakte`, `waad`, `weibo`, `windowslive`, `wordpress`, `yahoo`, `yammer`, `yandex`.
 * `options` - (Optional) List(Resource). Configuration settings for connection options. For details, see [Options](#options).
 * `enabled_clients` - (Optional) Set(String). IDs of the clients for which the connection is enabled. If not specified, no clients are enabled.
 * `realms` - (Optional) List(String). Defines the realms for which the connection will be used (i.e., email domains). If not specified, the connection name is added as the realm.