Feat: Add configurable webhook body/headers

[nRamstedt]

Extends the webhook handler to support configurable request body and headers. The configurable request body (JSON) allows placeholders ({{field}} / {{*}}) for field interpolation using dot-notation, example:

{
    "message": "{{message}}",
    "name": "{{user.name}}",
    "wants_feedback": "{{wants_feedback}}",
    "email": "{{email}}",
    "phone": "{{phone}}",
    "all": "{{*}}"
}

Placeholder values will match ACF field names or output all fields when using {{*}} .

Enable WP_DEBUG_LOG and set MODULARITY_FRONTEND_FORM_LOG_LEVEL to "debug" to output errors to the log.

Changes:

• add helsingborg-stad/psr-logger package
• add adbario/php-dot-notation package
• add loggning decorators for handler and handler result
• env vars: configure logger using "MODULARITY_FRONTEND_FORM_LOG_LEVEL" or "APP_LOG_LEVEL" (set to error, info, debug etc based on psr log levels)
• add hydrator interface and implementation for field interpolation (depends on adbario/php-dot-notation)
• add ACF field json validation for request body field
• add loggning documentation to README.md