Adjust linking for TS provider

[ionut-arm]

The current state of build.rs is centred on the idea that the TS provider will be used for local testing, with in-process Trusted Services. The service is thus linked to Mbed Crypto (which provides the crypto implementation) and to libstdc++.

In order to prepare for a state in which the TS provider is actually used as intended, with the TS running in a TEE, we only need to dynamically link against libts. It's worth checking, however, if the dependency on libstdc++ is required still. This involves making some of the dependencies conditional on, potentially, a compile-time flag or feature which indicates that the build is meant for testing - the CI should continue to work as it currently does.

[ionut-arm]

I'm currently in the process of updating our TS dependency from the upstream sources (see #461) but it seems they've release a whole bunch of new commits today. From a very brief skim, it seems the commits add some fixes that allow #406 to finally work, but also a big API change in the form of moving from an old version of the PSA Crypto API to v1.0 (at least). This means some of the calls will need changing and possibly also the interaction with the key info manager.

I'll be pulling in these changes through #406 after #461 is merged, and then look at fixing what this issue was raised for.