Add configs to disable unused APIs

[flynd]

Add configs to disable key generation, signature creation, and/or signature verification to reduce the library size when not needing only one or two of these.
Also adds a config to disable all but the internal APIs, allowing for example to build only crypto_sign_verify_internal() and exclude everything else.

Resolves #941

[mkannwischer]

Thanks @flynd. We will need a way to test the configuration options in CI.
Adding a new example based on the monobuild example seems to be the easiest path.

[flynd]

Thanks @flynd. We will need a way to test the configuration options in CI. Adding a new example based on the monobuild example seems to be the easiest path.

I started looking at this, but the standard examples create a signature and then verifies it. When building without signature creation the example code needs to do something else so I'm not sure what would be appropriate here.

[mkannwischer]

Thanks @flynd. We will need a way to test the configuration options in CI. Adding a new example based on the monobuild example seems to be the easiest path.

I started looking at this, but the standard examples create a signature and then verifies it. When building without signature creation the example code needs to do something else so I'm not sure what would be appropriate here.

We already have https://github.com/pq-code-package/mldsa-native/blob/main/examples/basic/expected_signatures.h. This could be extended also with keys, then you could implement keygen, sign, and verify separately.
Probably we do not want to touch existing examples as it would make them hard to understand, but for a new example, this should be fine.
I can take a look at some point, but this month is rather busy.

[flynd]

@mkannwischer : I've added an example that is pretty close to the actual configuration I'm using (except I omitted MLD_CONFIG_REDUCE_RAM in the example).