Filter all to both config && Cargo .tomls

[workingjubilee]

Our support for a given Postgres version, for an extension crate, is informed by two things:

• The setting to find pg_configs, defaulting to "${PGRX_HOME}/config.toml"
• The crate's Cargo.toml for a given pgrx "pg{major}" feature.

Interpret cargo-pgrx {start,stop,test} all to act only on versions for which both of these are present. We will simply filter silently without note because there are some nuances for when we should (not) emit errors that I am not entirely settled on how to decide, but ultimately this allows e.g. an extension like PL/Rust that never supported the same version minimum as pgrx to still use cargo pgrx test all, and causes less confusing errors when config.toml includes out-of-date versions unless those are also in Cargo.toml.

Note this doesn't prevent us from emitting errors: instead the error moves to when building the pgrx library. Currently it is first caught by cargo when the extension's Cargo.toml doesn't include the features to for the request from cargo-pgrx, but this is a hidden behavior of cargo-pgrx. The failed build of the library is easier to diagnose.

Closes #1421

[eeeebbbbrrrr]

LGTM!