add support for sha256 container image specifications

[adamf-db]

What problem does this PR solve?

The operator doesn't work when the image spec includes a sha256 image reference.

For security reasons, I much prefer to use the sha256 method of retrieving container images, rather then tags. Eg, we want to do pingcap/tidb@sha256:deadbeef.... instead of pingcap/tidb:v7.1.0. However, this breaks the operator because the operator tests if it can set labels in https://github.com/pingcap/tidb-operator/blob/master/pkg/manager/member/tidb_member_manager.go#L1142 by checking if the version extracted from the container image spec is greater than v6.3.0.

When I use the sha256 method of container image specification this code reports that we can't set labels and bails on all the code after that check.

This change makes it so that any valid sha256 is considered a greater version number then any other, similar to the logic for handling latest or nightly.

What is changed and how does it work?

The logic in the semantic version comparison is changed to support sha256 values. A value is consider a sha256 if hex.DecodeString returns no error and the length of the string is 64.

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:
  There's a potential that people are using 6.3.0 or early with sha256 container image specs.

Related changes

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

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

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[sre-bot]

All committers have signed the CLA.

[ti-chi-bot]

Welcome @adamf-db! It looks like this is your first PR to pingcap/tidb-operator 🎉

[csuzhangxc]

@adamf-db how about adding version stings as #5213