Add read/write support for service tokens and update CRUD examples in docs to use service tokens

[dangtony98]

Description 📣

This PR does a few things:

• Add read/write support for service tokens; this means that service tokens can now be included in Authorization headers for CRUD secret operations (minus the batch operation since that one will require more future refactoring).
• Re-write documentation to use service tokens to deal with CRUD secret operations. The previous approach with API keys and self-decryption of private keys was cumbersome and prone to error for users; service tokens simplify dealing with CRUD secret operations because the service token itself carries a client-side generated key to encrypt/decrypt its copy of the project (vault) key.

Type ✨

• Bug fix

• New feature

• Breaking change

• Documentation

• I have read the contributing guide, agreed and acknowledged the code of conduct. 📝