fix: Missing required fields cause panic

[cwurm]

The validation logic sometimes does not catch missing required fields, resulting in a panic. This updates the schema generation to fix it.

E.g. the following config:

services:
- host: httpbin.org
  #name: service
  routes:
  - headers:
      my-header:
      - httpbin.org
    #name: route
    plugins:
    - name: key-auth
consumers:
- keyauth_credentials:
  - key: foo
  #username: consumer
upstreams:
- hash_on: none
  #name: upstream

will cause segfaults like the following for any of the commented out lines above:

$ deck diff -s kong.yaml
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x141b947]

goroutine 7 [running]:
github.com/hbagdi/deck/file.(*stateBuilder).services(0xc0003cfb28)
	/home/hbagdi/deck/file/builder.go:392 +0x5d7
github.com/hbagdi/deck/file.(*stateBuilder).build(0xc0003cfb28, 0x0, 0x0, 0x0)
	/home/hbagdi/deck/file/builder.go:53 +0xfa
github.com/hbagdi/deck/file.Get(0xc000388000, 0xc00006d200, 0x1, 0x5, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/hbagdi/deck/file/reader.go:51 +0x130
github.com/hbagdi/deck/cmd.syncMain(0xc00005c060, 0x1, 0x1, 0xc0000f9d01, 0xa, 0x0, 0x0)
	/home/hbagdi/deck/cmd/common.go:102 +0x325
github.com/hbagdi/deck/cmd.glob..func1(0x1ad8440, 0xc00000c0e0, 0x0, 0x2, 0x0, 0x0)
	/home/hbagdi/deck/cmd/diff.go:25 +0x56
github.com/spf13/cobra.(*Command).execute(0x1ad8440, 0xc00000c0a0, 0x2, 0x2, 0x1ad8440, 0xc00000c0a0)
	/home/hbagdi/go/pkg/mod/github.com/spf13/cobra@v0.0.7/command.go:838 +0x453
github.com/spf13/cobra.(*Command).ExecuteC(0x1ad79c0, 0x0, 0x0, 0x0)
	/home/hbagdi/go/pkg/mod/github.com/spf13/cobra@v0.0.7/command.go:943 +0x317
github.com/spf13/cobra.(*Command).Execute(...)
	/home/hbagdi/go/pkg/mod/github.com/spf13/cobra@v0.0.7/command.go:883
github.com/hbagdi/deck/cmd.Execute.func2(0xc00005c010, 0xc000028110)
	/home/hbagdi/deck/cmd/root.go:59 +0x2d
created by github.com/hbagdi/deck/cmd.Execute
	/home/hbagdi/deck/cmd/root.go:58 +0xba

Also switches the order in which one of two fields are required to have id last so it gives a more meaningful error message - only the first is displayed, e.g.:

$ ./deck diff -s kong.yaml
Error: reading file: validating file content: 7 errors occurred:
	upstreams.0: name is required
	services.0: Must validate at least one schema (anyOf)
	services.0: name is required
	services.0.routes.0: Must validate at least one schema (anyOf)
	services.0.routes.0: name is required
	consumers.0: Must validate at least one schema (anyOf)
	consumers.0: username is required

[hbagdi]

Good catch, thanks for the clean patch.