enable customizing mydumper options in CRD backup (#2407)

[sre-bot]

cherry-pick #2407 to release-1.1

---

What problem does this PR solve?

This PR resolve #2391

What is changed and how does it work?

Support specifying mydumper options in the backup which can be used during backup with mydumper.
The mydumper options can refer to this doc

backup CR example:

---
apiVersion: pingcap.com/v1alpha1
kind: Backup
metadata:
  name: demo1-backup-s3
  namespace: test1
spec:
  from:
    host: demo1-tidb
    port: 4000
    user: root
    secretName: backup-demo1-tidb-secret
  s3:
    provider: aws
    bucket: test1-demo1
    region: us-west-2
    secretName: aws-secret
  mydumper:
    options:
    - --threads=16
    - --rows=10000
    - --skip-tz-utc
  storageClassName: local-storage
  storageSize: 1Gi

Check List

Tests

• Unit test
• E2E test
• Stability test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has Go code change

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Does this PR introduce a user-facing change?:

Support customizing `mydumper` options in Backup CR

[DanielZhangQD]

LGTM

[DanielZhangQD]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for:

• 2451
• 2449

[sre-bot]

/run-all-tests