Implement support for warnings

Elasticsearch sends query execution-related warnings in HTTP `Warning` headers. These need to be read and made available to the client as stacked diagnostics ([SQLGetDiagRec](https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlgetdiagrec-function)/[SQLGetDiagField](https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlgetdiagfield-function)).