feat(bigtable): Hot backups

[bhshkh]

Fixes: b/365774730

More about hot backups: https://cloud.google.com/bigtable/docs/backups#hot-backups

Generated protos have 2 new fields in the Backup struct. These fields are being added to various methods in this PR:

google-cloud-go/bigtable/admin/apiv2/adminpb/table.pb.go

Lines 1390 to 1400 in 649c075

	// Indicates the backup type of the backup.
	BackupType Backup_BackupType `protobuf:"varint,11,opt,name=backup_type,json=backupType,proto3,enum=google.bigtable.admin.v2.Backup_BackupType" json:"backup_type,omitempty"`
	// The time at which the hot backup will be converted to a standard backup.
	// Once the `hot_to_standard_time` has passed, Cloud Bigtable will convert the
	// hot backup to a standard backup. This value must be greater than the backup
	// creation time by:
	// - At least 24 hours
	//
	// This field only applies for hot backups. When creating or updating a
	// standard backup, attempting to set this field will fail the request.
	HotToStandardTime *timestamppb.Timestamp `protobuf:"bytes,12,opt,name=hot_to_standard_time,json=hotToStandardTime,proto3" json:"hot_to_standard_time,omitempty"`

---

To create a hot backup that expires after 4 days:

hotToStdTime := 
adminClient.CreateBackupWithOptions(ctx, "TableID", "Cluster", "BkpName",
		time.Now().Add(24 * 4 * time.Hour), HotToStandardBackup(time.Now().Add(2 * 24 * time.Hour)))

or without specifying HotToStandardTime i.e. hot backup does not get converted to standard backup

hotToStdTime := time.Now()
adminClient.CreateBackupWithOptions(ctx, "TableID", "Cluster", "BkpName",
		time.Now().Add(24 * 4 * time.Hour), HotBackup())

---

To update hot_to_standard_time of hot backup:

adminClient.UpdateBackupHotToStandardTime(ctx, "Cluster", "BkpName", &hotToStdTime)

To remove hot_to_standard_time of hot backup:

adminClient.UpdateBackupHotToStandardTime(ctx, "Cluster", "BkpName", nil)

[DerekLeeCS]

overall logic looks good, just have a few suggestions for the implementation

[shollyman]

I think my main question here for the option style here is how complex you expect the options to become. I see only a single option with two behavior fields, so struct-based options that satisfy the apply() contract may be more than you need.

By way of comparison, if the contract is simplified to the core backupOptions and some funcs that can modify it:

type BackupOption func(*backupOptions)

func WithHotBackupOption(t time.Time) BackupOption {
  return func(bo *backupOptions) {
     bo.backupType = blah
     bo.hotToStandardTime = blah
  }
}

func WithExpiration(t time.Time) BackupOption {
  return func(bo *backupOptions) {
     bo.expireTime = blah
  }
}

[bhshkh]

Thanks for the review. I have updated the code.

[bhshkh]

@DerekLeeCS , please review. I have updated the code as per review comments

[bhshkh]

@shollyman @gkevinzheng, please review. I have updated the code as per review comments

[DerekLeeCS]

thanks for the changes!