COS Backend Support `security_token` argument

The Tencentcloud Credential support temporary access by passing SessionToken. Here is the struct.

type AuthorizationTransport struct {
	SecretID     string
	SecretKey    string
	SessionToken string
	rwLocker     sync.RWMutex
	Expire    time.Duration
	Transport http.RoundTripper

The SessionToken is an optional field which generated by invoking Tencentcloud API AssumeRole, but right now this argument was not defined in internal/backend/remote-state/cos/backend.go, means user can only configure permanently credential.

terraform {

  backend "cos" {
    region = "ap-guangzhou"
    bucket = "tf-state-1234567890"
    prefix = "terraform/state"
    secret_id  = "xx" # Indicates `SecretID` - Secret id of Tencent Cloud.
    secret_key = "xxx" # Indicates `SecretKey` - Secret key of Tencent Cloud.
    security_token = "xxx" # (not support) Indicates `SessionToken` - Security Token of Tencent Cloud for temporary access credentials.

Here is the example pull request: