Implement delete target backup for Greenplum

[usernamedt]

I propose to add the delete target command support to WAL-G for Greenplum.

wal-g delete target [FIND_FULL] backup_name | --target-user-data <data> [flags]

Examples:
  target base_0000000100000000000000C4  delete base backup by name
  target --target-user-data "{ \"x\": [3], \"y\": 4 }"  delete backup specified by user data
  target base_0000000100000000000000C9_D_0000000100000000000000C4       delete delta backup and all dependant delta backups 
  target FIND_FULL base_0000000100000000000000C9_D_0000000100000000000000C4     delete delta backup and all delta backups with the same base backup