DBaaS: implement migration status command

// test code
    res, err := exo.GetDatabaseMigrationStatus(ctx, "ch-gva-2", "test1")
    if err != nil {
        log.Fatal(err)
    }

    spew.Dump(res)

// result test1
(*v2.DatabaseMigrationStatus)(0xc0002b6040)({
 Details: ([]v2.DatabaseMigrationStatusDetails) {
 },
 Error: (*string)(0xc0004bc290)((len=54) "Failed to run query on source db: 'connection refused'"),
 MasterLastIoSecondsAgo: (*int64)(<nil>),
 MasterLinkStatus: (*v2.DatabaseMigrationRedisMasterLinkStatus)(<nil>),
 Method: (*string)(0xc0004bc2b0)(""),
 Status: (*string)(0xc0004bc2a0)((len=4) "done")
})

// result test2
(*v2.DatabaseMigrationStatus)(0xc000120100)({
 Details: ([]v2.DatabaseMigrationStatusDetails) (len=1 cap=1) {
  (v2.DatabaseMigrationStatusDetails) {
   DBname: (*string)(0xc0004c22b0)((len=9) "defaultdb"),
   Error: (*string)(<nil>),
   Method: (*string)(0xc0004c22d0)((len=7) "pg_dump"),
   Status: (*v2.DatabaseMigrationStatusDetailsStatus)(0xc0004c22c0)((len=4) "done")
  }
 },
 Error: (*string)(<nil>),
 MasterLastIoSecondsAgo: (*int64)(<nil>),
 MasterLinkStatus: (*v2.DatabaseMigrationRedisMasterLinkStatus)(<nil>),
 Method: (*string)(0xc0004c22f0)((len=7) "pg_dump"),
 Status: (*string)(0xc0004c22e0)((len=4) "done")
})
exoscale / egoscale

DBaaS: implement migration status command #552
