Connection Pooling Feature with Tiproxy

## Feature Request

**Is your feature request related to a problem? Please describe:**
We have been using tiproxy in production, it works well. For one of our client the application doesnt support connection pooling as its legacy PHP application, which makes connection handling very hectic and cumbersome

**Describe the feature you'd like:**
We would love to have connection pool and multiplexing enabled as a part of Tiproxy

**Describe alternatives you've considered:**
we tried external proxy such as proxysql which comes with inbuilt connection pooling

**Teachability, Documentation, Adoption, Migration Strategy:**
This would help with the connection handling as a whole and increase the adoption of tiproxy