feat(auth): expose auth placeholders as empty collection variables on import (closes #894)

[Mrsandeep27]

Context

Closes #894.

When the imported OpenAPI spec declares a security scheme, `getAuthHelper` produces an auth block whose values template Postman variables:

```js
{ type: 'basic', basic: [
{ key: 'username', value: '{{basicAuthUsername}}' },
{ key: 'password', value: '{{basicAuthPassword}}' }
]}
```

…but those variables were never pushed into `collection.variable`. On first send the user hit an "unresolved variable" warning and had to hunt for where to put credentials — while `baseUrl` (which is treated the same way) already appears in the Variables tab waiting to be filled.

Change

Mirror the existing `baseUrl` flow. After `generatedStore.collection.auth = authHelper` is set in `schemapack.js`, scan the helper for `{{identifier}}` placeholders and add an empty collection-level `Variable` for each unique one.

```js
if (authHelper) {
schemaUtils.getAuthCollectionVariables(authHelper).forEach((element) => {
generatedStore.collection.variables.add(element);
});
}
```

New helper `schemaUtils.getAuthCollectionVariables(authHelper)` recursively walks the helper payload (skipping the `type` field), uses `/{{\s*([A-Za-z0-9_]+)\s*}}/g` to extract placeholder names, de-duplicates with a `Set`, and returns `Variable` objects with empty values.

Design notes:

• Works uniformly for every scheme that `getAuthHelper` produces — basic, bearer, digest, apiKey, oauth2, oauth1, hawk, awsv4, ntlm, edgegrid — without any per-scheme mapping. Whatever placeholder `getAuthHelper` emits is what gets surfaced.
• No-op for `{ type: 'noauth' }` and for helpers whose values contain no `{{…}}` tokens (e.g. a future helper that inlines literal values).
• Values are intentionally empty strings, matching the baseUrl convention for user-supplied values.

Test plan

Extended `test/unit/convertV2.test.js` → 'Should convert a collection and set basic auth placeholder as variable' to additionally assert:

```js
const variableKeys = conversionResult.output[0].data.variable.map((v) => v.key);
expect(variableKeys).to.include('basicAuthUsername');
expect(variableKeys).to.include('basicAuthPassword');
// both should be empty strings
```

• `npm test` — all existing tests pass; the extended basic-auth case now also verifies the new variables.
• Import an OpenAPI spec with a `bearer` scheme → confirm `bearerToken` appears in the Variables tab (empty).
• Import with `digest` / `apiKey` / `oauth2` → confirm the respective placeholders are exposed.
• Import a spec with no security → `collection.variable` contains only `baseUrl` + any server variables, unchanged.

Out of scope (for follow-up)

• The 13+ other test cases covering non-basic auth types in `convertV2.test.js` could each get the same assertion block. Happy to add those in this PR if preferred — left them untouched to keep the diff surgical, since the new helper is generic and exercised through the basic-auth path.

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