Eliminate keyring, renv and targets package requirements

[anthonysena]

As noted in #102 and through feedback from multiple network studies, the required use of the keyring package inside of Strategus is problematic since it requires operating system dependencies and often user intervention to enter credentials.

The main goal with using keyring inside of Strategus was to facilitate a secure mechanism to store credentials. With some of the ideas put forward in other issues (#128), we may be able to find alternative approaches for credential management to eliminate the need for the keyring package dependency. For example, if a targets script is generated by Strategus the the credential management can also live outside of the Strategus package. Alternatively, we might adopt a file-based encryption approach that is agnostic to the operating system.

Following along these lines, part of the reason why we are using keyring was due to the way in which we are dynamically generating a targets script and running modules via renv. We should remove these package dependencies and allow for a user to use renv and optionally targets outside of the scope of Strategus per #29 .