* Add support for cloud sync, specifically GCP
This adds generic support for sync to cloud services, with specific
spuport for GCP. Adding others -- so long as they support a
compare-and-set operation -- should be comparatively straightforward.
The cloud support includes cleanup of unnecessary data, and should keep
total space usage roughly proportional to the number of tasks.
Co-authored-by: ryneeverett <ryneeverett@gmail.com>
In #3118 @ryneeverett mentioned that "key" suggests that this is a
secret, when in truth it's just a user identifier. So "ID" is a better
word for it than "key".
This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
* `debug.tls`
* `taskd.ca`
* `taskd.certificate`
* `taskd.ciphers`
* `taskd.credentials`
* `taskd.key`
* `taskd.server`
* `taskd.trust`
