Support cryptography module

[InfoHunter]

This pull request implements the rusty_vault::modules::crypto module and relevant cryptography adaptor mechanism. In every build of RustyVault, users can select one of the supported adaptors as the real cryptography operations provider in RustyVault.

A set of new feature names are added in Cargo and they can be specified during cargo build by:

cargo build --features crypto_adaptor_tongsuo

or 

cargo build --features crypto_adaptor_openssl

If no adaptor is explicitly configured, then the OpenSSL adaptor is selected by default - for the compatibility considerations.

[InfoHunter]

It seems Cargo doesn't support dynamic patching for dependencies. So if Tongsuo is selected as the underlying adaptor, one must run the following command to patch openssl and openssl-sys to the rust-tongsuo version:

cargo build --features crypto_adaptor_tongsuo \
    --no-default-features \
    --config 'patch.crates-io.openssl.git="https://github.com/Tongsuo-Project/rust-tongsuo.git"'\
    --config 'patch.crates-io.openssl-sys.git="https://github.com/Tongsuo-Project/rust-tongsuo.git"'

Or, just uncomment the following lines in Cargo.toml:

#[patch.crates-io]
#openssl = { git = "https://github.com/Tongsuo-Project/rust-tongsuo.git" }
#openssl-sys = { git = "https://github.com/Tongsuo-Project/rust-tongsuo.git" }

and then:

cargo build --features crypto_adaptor_tongsuo --no-default-features

[InfoHunter]

To use Tongsuo as the underlying cryptography library, the user needs to set a OPENSSL_DIR=/path/to/tongsuo/install/directory, to indicate where Tongsuo is installed. Currently only Tonsguo 8.4 and later are supported.

[InfoHunter]

some in-code document snips:

[InfoHunter]

Support of public key and digest algorithms will be in a separate pull request.

[InfoHunter]

All have been resolved.

[InfoHunter]

Able to merge subject to CIs are green

[wa5i]

LGTM