Read GPG passphrase from an environment variable

[nnarain-cpr]

The gnupg library requires users to supply the passphrase for their key as a string into the decrypt function. There currently is no way to supply a passphrase through the rosbag command line tool, meaning only keys without passphrases can be used to decrypt encrypted bags.

This PR enables fetching the passphrase from the environment variable ROSBAG_GPG_PASSPHRASE.

[mikepurvis]

Thanks for addressing this. Down the road, we need to figure out how to get the Python API to rosbag transparently using the gpg agent for the passphrase so that it works the same as how the C++ API does. But this is fine in the short term.

[mikepurvis]

@dirk-thomas Any concerns here? I doubt anyone else was using gpg keys with the passphrase set to clearpath, so I'd expect this is a safe change.

[dirk-thomas]

Thanks for the patch.