Remove the `proxy_pass_request_body off` directive, which unintentionally broke
OIDC client authentication using the POST body method (`client_secret_post`).

Previously, when `$oidc_client_auth_method` was set to "client_secret_post"
the `generateTokenRequestParams()` function correctly formatted the POST
request and sent it via `r.subrequest` to the internal `/_token` location.
However, the `proxy_pass_request_body off` directive caused the POST request
to reach `$oidc_token_endpoint` with a valid Content-Length header but an
empty body. This led to a timeout as the OP token endpoint closed the connection.

Users encountered the error: "NGINX / OpenID Connect login failure."

This commit restores functionality by ensuring the request body is passed
to the token endpoint while retaining header exclusion to prevent CORS issues.

…n handling.

- Switched from callbacks to async/await for clearer, more maintainable code.
- Broke up the monolithic code into smaller and modular functions.
- Refined id token validation logic.
- Changed the internal /_id_token_validation location to /_token_validation.
- Minimum required njs version is 0.7.0 now.

Implement OpenID Connect Front-Channel Logout 1.0 specification:
- Add default /front_channel_logout location that handles logout requests
- Both sid and iss parameters must be present
- Issuer verification against iss claim in ID token

Reference: https://openid.net/specs/openid-connect-frontchannel-1_0.html

- Implemented unified function for error handling.
- Each error is assigned its own identifier, generated by using the first
8 chars of the $request_id variable.
- Added support for JSON log output. This is controlled by the
$oidc_log_format variable, which must be set to 'json'.
- Added support for stack trace output. This is enabled by the
$oidc_debug variable, which must have any non-empty value.
If this variable is defined, the $internal_error_message variable
is overwritten with the text of the last error and returned to
the User Agent - so use this only for debugging!