Demonstrate usage of macOS keychain to store passwords

[ktf]

In case you are interested, this is what I use to store the password in the macos keychain. While it should not be complicated, my haskell-fu is not large enough to support both linux and macOS in the same sourcecode.

[pdobsan]

Thank you for the information. Might use it later.

[pdobsan]

I have significantly refactored oama by moving the low level details of
dealing with secrets into two external packages. They provide the same high
level interface but the internal implementation is different. I incorporated
your solution of using the security utility in macOS to deal with keyring into
one of these external packages (secret-tools).

You can build oama in macOS using this method by issuing:

make build-with-tools

For more details see the Readme. The corresponding external package used in this
case is https://github.com/pdobsan/secret-tools

Would you mind to test the new oama in your macOS environment?

[pdobsan]

Meanwhile I moved secret management into the Crypto.Manager internal module.
There are no external packages any more.

cabal build or make build-with-tools should work in macOS.