Upgrade in-toto to gain full cross-platform release signing support

[ofek]

Motivation

secure-systems-lab/securesystemslib#156
in-toto/in-toto#229

[codecov-io]

Codecov Report

Merging #2315 into master will decrease coverage by 11.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2315       +/-   ##
===========================================
- Coverage    85.9%   74.84%   -11.06%     
===========================================
  Files         196       17      -179     
  Lines       13515      493    -13022     
  Branches     1453       57     -1396     
===========================================
- Hits        11610      369    -11241     
+ Misses       1488      105     -1383     
+ Partials      417       19      -398

Flag	Coverage Δ
#activemq_xml	?
#agent_metrics	?
#apache	?
#btrfs	?
#cacti	?
#cassandra_nodetool	?
#ceph	?
#cisco_aci	?
#cockroachdb	?
#consul	?
#couch	?
#couchbase	?
#datadog_checks_base	?
#datadog_checks_dev	74.84% <ø> (ø)	⬆️
#directory	?
#disk	?
#dns_check	?
#ecs_fargate	?
#envoy	?
#etcd	?
#fluentd	?
#gearmand	?
#go_expvar	?
#gunicorn	?
#hdfs_datanode	?
#hdfs_namenode	?
#http_check	?
#istio	?
#kube_dns	?
#kube_proxy	?
#kubelet	?