feat: Add 1Password provider support

[dirathea]

Description

This PR adds support for 1Password as a secret provider, allowing users to retrieve secrets from 1Password vaults using the 1Password Connect SDK.

Features

• 1Password Connect SDK Integration: Uses the official 1Password Go SDK for secure secret retrieval
• Flexible Reference Format: Supports multiple reference formats:
  • Single field: op://vault/item/field or op://vault/item/section/field
  • Whole section: op://vault/item/section (fetches all fields from a section)
  • Whole item: op://vault/item (fetches all fields from an entire item)
• Section Prefix Control: Optional use_section_prefix configuration to control whether section names are used as prefixes for field keys
• Comprehensive Testing: Includes end-to-end tests for various scenarios

Configuration

The provider requires:

• ref: 1Password secret reference in the format op://<vault>/<item>/[section/]<field>
• OP_SERVICE_ACCOUNT_TOKEN: Environment variable for 1Password Connect API authentication

Documentation

• Updated CONFIGURATION.md with detailed 1Password provider documentation
• Updated README.md to include 1Password in the list of supported providers

Testing

• End-to-end tests included in tests/end2end/onepassword_test.go
• Test helpers for 1Password Connect setup

Closes #8