[WIP] Adds ltree string type

[auyer]

This adds ltree type as a string, using commit as guide 9ae1ecb.
ltree is available in the driver since sfackler/rust-postgres#389
Works with all protocols, but has an issue with Binary:

When I inspect the data in the shell, the dataframe that came from Postgres shows this hex prefix.

I'm looking for help delivering this, and other Postgres Types. Thanks!
Fixes: #381

[wangxiaoying]

Hi @auyer , thanks for the PR and it looks great!

For the error, I tried to print output value from rust-postgres directly and it shows the return value using copy_out with BINARY has this single hex char prefix for ltree type:

which does not happen in copy_our with CSV or query_raw in rust-postgres:

I think this is an issue from rust-postgres, and we can report this to them. In the meanwhile, we can remove this ltree type test from test_postgres_types_binary for now and add a comment in our document of the postgres type mapping (which corresponds to the markdown file at /docs/databases/postgres.md#postgres-pandas-type-mapping).

[auyer]

Sounds great !
I will open the issue there and make the changes to this PR.
Tomorrow I plan to start work on Arrays ! Any advice is welcome 😁