cliccl/debug_backup.go: refactor backup inspection tool

[Elliebababa]

Previously, cli backup inspection tool was incorporated
in cliccl/load.go. Both the command and syntax are
confusing in terms of backup inspection.

This patch refactors the naming and syntax of the tool
and hang the tool under cockroach debug, so that
it makes more sense when using it to debug backups.

Release note (cli change): Backup inspection used to be done via
cockroach load show .., which was confusing to users with
ambiguous verbs in the command chain. We refactor the syntax of
tool and make the syntax more clear and indicative for users to
debug backups. The changes are:
load show summary <backup_url> ->
debug backup show <backup_url>

$ ./cockroach debug backup show $AWS_BACKUP_PATH
{
	"StartTime": "1970-01-01T00:00:00Z",
	"EndTime": "2021-04-05T22:02:25Z",
	"DataSize": "110 KiB",
	"Rows": 224,
	"IndexEntries": 324,
	"FormatVersion": 1,
	"ClusterID": "7c1b9ceb-d05a-4879-b4a4-886ace456953",
	"NodeID": 0,
	"BuildInfo": "CockroachDB CCL v21.1.0-alpha.3-1790-g25dc628337-dirty (x86_64-apple-darwin19.6.0, built 2021/03/29 18:50:47, go1.15.7)",
	"Files": [
		{
			"Path": "647462079350243329.sst",
			"Span": "/Table/4/{1-2}",
			"DataSize": "99 B",
			"IndexEntries": 0,
			"Rows": 2
		},
	],
	"Spans": "[/Table/4/{1-2}]",
	"DatabaseDescriptors": {
		"1": "system"
	},
	"TableDescriptors": {
		"53": "demodb.public.demo",
		"54": "demodb.public.foo"
	},
	"TypeDescriptors": {},
	"SchemaDescriptors": {
		"29": "public"
	}
}

load show incremental <backup_url> ->
debug backup list-incremental <backup_url>

$ ./cockroach debug backup list-incremental $AWS_BACKUP_PATH
              path             |      start time      |       end time
-------------------------------+----------------------+-----------------------
  /mybackup                    | -                    | 2021-04-09T20:22:59Z
  /mybackup/20210409/202303.20 | 2021-04-09T20:22:59Z | 2021-04-09T20:23:03Z
  /mybackup/20210409/202305.97 | 2021-04-09T20:23:03Z | 2021-04-09T20:23:05Z
(3 rows)

load show backups <collection_url> ->
debug backup list-backups <collection_url>

$ ./cockroach debug backup list-backups $AWS_COLLECTION_PATH
           path
--------------------------
  ./2021/04/09-202455.98
  ./2021/04/09-202522.59
  ./2021/04/09-202526.93
(3 rows)

load show data <table_name> <backup_url> ->
debug backup export <backup_url> --table=<table_name> [--destination=</path/to/export>]

$ ./cockroach debug backup export $AWS_BACKUP_PATH --table=demodb.public.demo
1,null,true
2,'ellie',null

[cockroach-teamcity]

This change is 

[Elliebababa]

bors r=pbardea

[craig]

Build succeeded:

• GitHub CI (Cockroach)