Add ZK BBS+-based selectively disclosable credentials (JPT)

[UMR1352]

Description of change

todo

Links to any relevant issues

Closes #144 Closes #1324

Type of change

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [x] Enhancement (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Fix

How the change has been tested

Added unit and integration tests, examples for both WASM and Rust targets.

Change checklist

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[wulfraem]

Also tests::test_jwk_storage::write_to_disk (in identity_stronghold) runs into an error on my machine:

$ cargo test tests::test_jwk_storage::write_to_disk -- --exact
    Finished test [unoptimized + debuginfo] target(s) in 0.15s
     Running unittests src/lib.rs (/Users/swo/projects/iota/identity.rs-2/target/debug/deps/identity_stronghold-2e61cbd91c5befb5)

running 1 test
test tests::test_jwk_storage::write_to_disk ... FAILED

failures:

---- tests::test_jwk_storage::write_to_disk stdout ----
thread 'tests::test_jwk_storage::write_to_disk' panicked at identity_stronghold/src/tests/test_jwk_storage.rs:97:3:
assertion failed: exists
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    tests::test_jwk_storage::write_to_disk

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 12 filtered out; finished in 0.02s

[wulfraem]

CI errors looks like to fail due to feature "bbs-plus" not being enabled in identity_stronghold.

Works locally if I start the tests with cargo test --features "bbs-plus". ^^