Issue Description:

Currently, ProxySQL enforces a strict subset of PostgreSQL connection parameters as per the official documentation, rejecting any undocumented parameters. However, actual PostgreSQL servers accept additional parameters (e.g., extra_float_digits) and apply them at the connection/session level. To align with PostgreSQL's behavior and improve compatibility, ProxySQL should:

• Accept all connection parameters provided by clients, not just the documented subset.
• Defer parameter validation until after successful authentication to optimize resource usage.

Proposed Changes:

1. Remove Parameter Whitelist Enforcement:

   • Current Behavior: ProxySQL validates parameters against a static list from PostgreSQL documentation and rejects unrecognized ones.
   • New Behavior: Accept all parameters sent by the client, mirroring PostgreSQL's permissive handling. For example, extra_float_digits or other session parameters/variables should be accepted and forwarded to the server.

2. Defer Parameter Validation Until Post-Authentication:

   • Current Behavior: Parameters are validated during the connection handshake, regardless of authentication success.
   • New Behavior: Validate and apply parameters only after successful authentication. This avoids wasting resources on invalid connections (e.g., failed logins).