feat: Define portable authentication type schemas for cross-connector credential sharing

## Summary

This issue tracks the work to define and implement portable authentication type schemas in the Python CDK that enable credential sharing across connectors (sources, destinations, and sonar integrations) that connect to the same API.

**Requested by:** AJ Steers (@aaronsteers)

## Background

Analysis of 693+ connectors across Airbyte OSS, Enterprise, and Sonar integrations identified 10 distinct authentication mechanisms that can be consolidated into standardized JSON schemas. The goal is to make authentication configuration portable across connector types for the same API.

## Objectives

1. Define JSON schemas for all identified authentication types
2. Build these schemas into the Python CDK
3. Provide a migration path for legacy connectors with differently-named authentication fields
4. Require connectors to declare transformation logic mapping legacy config to new schemas (for major version bump)

## Proposed Authentication Type Schemas

Click to expand: Authentication Type Definitions

### 1. OAuth 2.0

```yaml
oauth2:
type: object
title: "OAuth 2.0"
required: [access_token]
properties:
access_token:
type: string
title: "Access Token"
airbyte_secret: true
refresh_token:
type: string
title: "Refresh Token"
airbyte_secret: true
client_id:
type: string
title: "Client ID"
airbyte_secret: true
client_secret:
type: string
title: "Client Secret"
airbyte_secret: true
token_expiry:
type: string
format: date-time
title: "Token Expiry"
```

### 2. Personal Access Token (PAT)

```yaml
personal_access_token:
type: object
title: "Personal Access Token"
required: [token]
properties:
token:
type: string
title: "Personal Access Token"
airbyte_secret: true
```

### 3. Basic Authentication

```yaml
basic_auth:
type: object
title: "Basic Authentication"
required: [username, password]
properties:
username:
type: string
title: "Username"
password:
type: string
title: "Password"
airbyte_secret: true
```

### 4. API Key

```yaml
api_key:
type: object
title: "API Key"
required: [key]
properties:
key:
type: string
title: "API Key"
airbyte_secret: true
header_name:
type: string
title: "Header Name"
default: "Authorization"
prefix:
type: string
title: "Prefix"
default: ""
```

### 5. Bearer Token

```yaml
bearer_token:
type: object
title: "Bearer Token"
required: [token]
properties:
token:
type: string
title: "Bearer Token"
airbyte_secret: true
```

### 6. Database Credentials

```yaml
database_credentials:
type: object
title: "Database Credentials"
required: [host, port, username, password]
properties:
host:
type: string
title: "Host"
port:
type: integer
title: "Port"
username:
type: string
title: "Username"
password:
type: string
title: "Password"
airbyte_secret: true
database:
type: string
title: "Database"
ssl_mode:
type: string
title: "SSL Mode"
enum: [disable, require, verify-ca, verify-full]
```

### 7. Cloud Provider IAM

```yaml
cloud_iam:
type: object
title: "Cloud Provider IAM"
properties:
provider:
type: string
enum: [aws, gcp, azure]
# AWS
aws_access_key_id:
type: string
airbyte_secret: true
aws_secret_access_key:
type: string
airbyte_secret: true
aws_role_arn:
type: string
# GCP
gcp_service_account_json:
type: string
airbyte_secret: true
# Azure
azure_tenant_id:
type: string
azure_client_id:
type: string
azure_client_secret:
type: string
airbyte_secret: true
```

### 8. JWT (JSON Web Token)

```yaml
jwt:
type: object
title: "JWT Authentication"
required: [private_key]
properties:
private_key:
type: string
title: "Private Key"
airbyte_secret: true
issuer:
type: string
title: "Issuer"
subject:
type: string
title: "Subject"
audience:
type: string
title: "Audience"
```

### 9. Session Token

```yaml
session_token:
type: object
title: "Session Token"
properties:
session_token:
type: string
title: "Session Token"
airbyte_secret: true
login_url:
type: string
title: "Login URL"
username:
type: string
password:
type: string
airbyte_secret: true
```

### 10. No Authentication

```yaml
no_auth:
type: object
title: "No Authentication"
properties: {}
```

## Implementation Plan

1. **Phase 1: Schema Definition**
- Define JSON schemas for all 10 authentication types
- Create validation utilities for each schema
- Add schema documentation

2. **Phase 2: CDK Integration**
- Add authentication schemas to CDK core
- Create base authenticator classes that accept canonical schemas
- Implement adapter pattern for schema transformation

3. **Phase 3: Migration Support**
- Define migration interface for legacy connectors
- Create helper functions for common field mappings
- Document migration requirements for major version bump

4. **Phase 4: Connector Updates**
- Update connectors to use new schemas (or declare adapters)
- Validate backward compatibility
- Release as major CDK version

## Related Analysis

This issue is informed by comprehensive authentication analysis across:
- 674 Airbyte OSS connectors
- 9 Airbyte Enterprise connectors
- 11 Sonar integrations

Key findings:
- 6 OAuth implementation patterns identified (client credentials, static refresh, single-use refresh with write-back, bearer passthrough, JWT service account, multiple options)
- 4 simplified categories: Identity-Based, Username/Password, Token/Key Handoff, No Auth
- 8 of 10 sonar connectors have identical auth support to traditional counterparts
- Field name normalization needed (e.g., Stripe: `api_key` vs `client_secret`)

## Devin Session

Link to Devin run: https://app.devin.ai/sessions/bf1ea66ad90247afb3f7156ffb2f05a7