Fix Client::getTime()

[TomA-R]

What/Why?

Fixes #298

• Updates Client::getTime() to expect time in milliseconds as that's the response from /time
• Adds Client::getStoreTime() which queries https://api.bigcommerce.com/stores/{{storeHash}}/v2/time. That requires authentication so this method can be used to verify that the client is configured correctly
• Update README to reflect changes

Testing

Updated ::getTime() I've updated the unit test to reflect what's actually returned by the API. Also I've run the steps provided to reproduce this issue, verifying that it no longer happens:

php > var_dump(\Bigcommerce\Api\Client::getTime());
php shell code:1:
class DateTime#6 (3) {
  public $date =>
  string(26) "2024-06-13 13:43:56.124000"
  public $timezone_type =>
  int(1)
  public $timezone =>
  string(6) "+00:00"
}

Added ::getStoreTime()

php > var_dump(\Bigcommerce\Api\Client::getStoreTime());
php shell code:1:
class DateTime#7 (3) {
  public $date =>
  string(26) "2024-06-14 09:01:35.000000"
  public $timezone_type =>
  int(1)
  public $timezone =>
  string(6) "+00:00"
}