[CDTOOL-1322] Add Support for DNS Zones and TSIG Keys

[rcaril]

Change summary

This PR adds support for Fastly DNS Zones and TSIG Keys.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

 make test TEST_ARGS="-run TestZoneCreate ./pkg/commands/dns/zone"
ok      github.com/fastly/cli/pkg/commands/dns/zone     1.096s
make test TEST_ARGS="-run TestZoneDescribe ./pkg/commands/dns/zone"
ok      github.com/fastly/cli/pkg/commands/dns/zone     1.087s
make test TEST_ARGS="-run TestZoneDelete ./pkg/commands/dns/zone"
ok      github.com/fastly/cli/pkg/commands/dns/zone     1.072s
 make test TEST_ARGS="-run TestZoneList ./pkg/commands/dns/zone"
ok      github.com/fastly/cli/pkg/commands/dns/zone     1.081s
make test TEST_ARGS="-run TestZoneUpdate ./pkg/commands/dns/zone"
ok      github.com/fastly/cli/pkg/commands/dns/zone     1.102s

 make test TEST_ARGS="-run TestTSIGKeyCreate ./pkg/commands/dns/tsigkey"
ok      github.com/fastly/cli/pkg/commands/dns/tsigkey  1.111s
make test TEST_ARGS="-run TestTSIGKeyDescribe ./pkg/commands/dns/tsigkey"
ok      github.com/fastly/cli/pkg/commands/dns/tsigkey  1.081s
 make test TEST_ARGS="-run TestTSIGKeyDelete ./pkg/commands/dns/tsigkey"
ok      github.com/fastly/cli/pkg/commands/dns/tsigkey  1.072s
 make test TEST_ARGS="-run TestTSIGKeyList ./pkg/commands/dns/tsigkey"
ok      github.com/fastly/cli/pkg/commands/dns/tsigkey  1.071s
make test TEST_ARGS="-run TestTSIGKeyUpdate ./pkg/commands/dns/tsigkey"
ok      github.com/fastly/cli/pkg/commands/dns/tsigkey  1.098s

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

None.

[philippschulte]

This looks good to me! Please just double check create/update for zones with --primary-description which requires --primary-address. Thank you!