Proposal for standardization of metadata keys of schema fields containing the SQL type string

### What feature or improvement would you like to see?

This proposal seeks to standardize the naming convention for commonly used metadata keys.

Specifically, we would like to see metadata keys on schemas which preserve the textual representation of the native warehouse type to be standardized. Currently, drivers are all inconsistently naming these keys (`SNOWFLAKE_TYPE` for Snowflake, `ADBC:postgresql:typname` for Postgres)

For instance, the BigQuery driver will use `BIGQUERY:type` as the key for a metadata field containing the native warehouse type.

By enforcing this as a standard, we will be achieve the following:
- Standardization around how to interpret common metadata keys
- Lossless operations relating to data types queried from ADBC