[feature] Add vercel support for assets retrieval

[Ice3man543]

Problem

Cloudlist lacked support for retrieving assets from Vercel, preventing users from enumerating their Vercel infrastructure alongside other cloud providers.

Solution

Added comprehensive Vercel provider support to cloudlist, enabling automated retrieval of Vercel deployments and domains.

Implemented Features

Supported Services:

• Deployments: Retrieves all deployments from Vercel projects with DNS names and public/private flags
• Domains: Retrieves production domain aliases associated with projects

API Integration:

• Full Vercel API v8 client implementation
• Bearer token authentication support
• Optional team ID support for team-scoped resources
• Proper error handling with detailed error messages
• JSON request/response handling

Configuration:

vercel:
  - id: vercel-account
    apiAccessToken: $VERCEL_API_TOKEN
    apiTeamID: team_xxx  # optional, for team resources
    services:
      - deployments
      - domains

Resource Enumeration:

• Lists all projects under the account/team
• Extracts deployment resources with DNS names and visibility status
• Extracts domain aliases configured for production environments
• Returns structured resource data compatible with cloudlist's schema

Implementation Details

• New provider package at pkg/providers/vercel/
• Integration with inventory system
• Proper service filtering and validation
• Context-aware resource retrieval

Implemented in PR #567