drush command fastly:purge:all does not work if used before the Admin UI purge is used

Problem/Motivation

If you install and configure the module, and then try to run `drush fastly:purge:all`, you get:

> Unable to purge/invalidate all site content on Fastly.

This is because the drush command checks the state of the purge credentials:

```
if ($this->state->getPurgeCredentialsState()) {
```

but that state is only set if the purge all command is invoked via the form/UI. So, if that form has never been used, the state is unset and `getPurgeCredentialsState()` returns falsey.

Steps to reproduce

Install Fastly.
Set service id and token (either as environment variable or setting).
Try to run `drush fastly:purge:all`

Proposed resolution

The solution is for the command to check the purge credentials state, and if it is undefined or missing, call `api-> public function validatePurgeCredentials()` and set the state accordingly and then continue with the command.

Remaining tasks

User interface changes

API changes

Data model changes