Add support for base64-encoded application logos

[ajilty]

Description

This PR adds support for uploading application logos using base64-encoded data instead of requiring local file paths. This enhancement provides better flexibility for automation and CI/CD workflows where managing local files is cumbersome.

Changes

Core Implementation

• New Field: Added logo_base64 field to all app resource schemas
• Validation: Created LogoBase64IsValid() function to validate base64 data and enforce 1MB size limit
• SDK Method: Implemented UploadApplicationLogoFromBase64() in sdk/v2_application.go
• Conflict Resolution: Made logo and logo_base64 mutually exclusive using ConflictsWith

Resources Updated

All 9 app resources now support logo_base64:

• okta_app_bookmark
• okta_app_auto_login
• okta_app_basic_auth
• okta_app_shared_credentials
• okta_app_swa
• okta_app_three_field
• okta_app_secure_password_store
• okta_app_oauth
• okta_app_saml

Documentation

• Auto-generated documentation includes the new logo_base64 field for all app resources
• Clear descriptions and examples provided

Usage Example

Before (File Path):

resource "okta_app_bookmark" "example" {
  label = "Example App"
  url   = "https://example.com"
  logo  = "/path/to/logo.png"
}

After (Base64):

resource "okta_app_bookmark" "example" {
  label       = "Example App"
  url         = "https://example.com"
  logo_base64 = filebase64("/path/to/logo.png")
}

Testing

• ✅ Code compiles successfully with go build
• ✅ Code formatted with go fmt
• ✅ Documentation generated with make docs
• ⚠️ Acceptance tests: Best effort implementation - would appreciate help running full test suite

Checklist

Following the contributing guidelines:

• Well-formed code following Go conventions
• Code formatted with go fmt
• Documentation updates included (auto-generated)
• Error handling with proper context messages
• Proper rollback on upload failure
• Uses existing SDK patterns and validation functions
• Maintains backward compatibility
• Mutually exclusive fields using ConflictsWith
• 1MB size limit enforced (matches existing logo field)

Benefits

1. Better CI/CD Integration: No need to manage temporary files in automated pipelines
2. Cleaner Workflows: Use Terraform's filebase64() function directly
3. Consistency: Similar pattern to other providers (e.g., AWS, Azure)
4. Backward Compatible: Existing logo field continues to work as before

Notes

This is my first contribution to the Okta Terraform provider. I've done my best to follow all contributing guidelines and would appreciate any feedback or suggestions for improvements!