ext/psycopg2: Implement BaseInstrumentor interface

[codeboten]

In order for users of psycopg2 to take advantage of opentelemetry-auto-instrumentation, the following needs to happen:

1. add a class implementing the BaseInstrumentor interface. here is an example for ext/pymysql
2. an entrypoint must be added to setup.cfg. here is an example for ext/pymsql
3. tests added to ensure the new instrumentor behaves as expected. here is an example for ext/pymysql
4. the documentation must be updated to reflect the new method of instrumenting psycopg2 applications
5. CHANGELOG.md must be updated

[cnnradams]

@codeboten can this be assigned to me?

[codeboten]

All yours