Expose conversion to WKT and PROJJSON

Relevant functions:

- [`proj_as_projjson`](https://proj.org/en/9.3/development/reference/functions.html#c.proj_as_projjson)
- [`proj_as_wkt`](https://proj.org/en/9.3/development/reference/functions.html#c.proj_as_wkt)

Would the right place to put these be as methods on the `Proj` struct?

Start by reading the Proj struct and the linked PROJ references for proj_as_projjson and proj_as_wkt. Determine how these functions should be exposed through the Rust API, then verify that both conversions are available on Proj and covered by the project's relevant tests.

Written by the indexing model from the issue text.