r/adbcdrivermanager: Expose SQL type mapping

For the DBI integration of ADBC via adbcdrivermanager (adbi), I need a way to determine the SQL type that corresponds to a given R type. Something like

```r
DBI::dbDataType(DBI::ANSI(), integer())
#> [1] "INT"
DBI::dbDataType(DBI::ANSI(), character())
#> [1] "TEXT"
```

As adbi is backend-agnostic, I cannot solve this issue as part of adbi, but need infrastructure to delegate such requests to. A first iteration (as discussed) could be a (doubly dispatched) generic in adbcdrivermanager that for a given combination of backend and R type returns the corresponding SQL type.

Start by reading the adbcdrivermanager DBI integration and the existing backend/type-dispatch entry points; no specific files or tests are named in the issue. Define the first-iteration generic for backend and R-type combinations, and consider it done when those requests return the corresponding SQL type for the supported combinations.

Written by the indexing model from the issue text.