Add delete transients

[Ninodevo]

Changes proposed in this PR

• Add option to delete transients
• Delete all transients on plugin activation

Testing

1. Create some action that will add some transients in the DB
2. Install and activate Safety net
3. The transients should be deleted

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1. Install and activate Safety net
2. Create some action that will add some transients in the DB
3. Go to Safety net options and use the button to delete all transients
4. The transients should be deleted

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1. Install and activate Safety net
2. Create some action that will add some transients in the DB
3. Use the CLI command wp safety-net delete-transients
4. The transients should be deleted

Mentions #137