CDH｜ Reorganize the sub-crates into module

[Xynnn007]

Due to #419 from @ChengyuZhu6 , we have a plan to publish some crates of guest-components to crates.io. But now we have the following crates

members = [
    "api-server-rest",
    "attestation-agent/app",
    "attestation-agent/lib",
    "attestation-agent/kbc",
    "attestation-agent/kbs_protocol",
    "attestation-agent/attester",
    "attestation-agent/deps/resource_uri",
    "attestation-agent/deps/crypto",
    "attestation-agent/deps/sev",
    "attestation-agent/coco_keyprovider",
    "attestation-agent/test-binaries",
    "confidential-data-hub/hub",
    "confidential-data-hub/kms",
    "confidential-data-hub/image",
    "confidential-data-hub/secret",
    "confidential-data-hub/storage",
    "image-rs",
    "ocicrypt-rs",
]

Some of them might have too universal name like image, and thus cause naming clash when cargo audit.

Due to suggestion from @mkulke , we are hoping to making some crates into modules.

cc @fitzthum @mythi

[mkulke]

• cdh: move image crate into hub/image module #812
• cdh: move storage and secret crates into cdh modules #814
• cdh: move kms crate to cdh module #816
• move everything from the hub hierarchy into the ./confidential-data-hub root folder

edit, there's also docs + some golang code in ./hub, so I think we can leave it at this.