add endorsement op but only allow legacy

[CoderZhi]

In this PR, a new field Op is introduced. OpLegacy, the default value, is for the current implementation of CandidateEndorsement action. And the others are new ops.

As it has been gated in action/protocol/staking/validations.go, only legacy actions are allowed. A follow up PR will add a hardfork to allow the new ops.

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• [] Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Code refactor or improvement
• [] Breaking change (fix or feature that would cause a new or changed behavior of existing functionality)
• [] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [] make test
• [] fullsync
• [] Other test (please specify)

Test Configuration:

• Firmware version:
• Hardware:
• Toolchain:
• SDK:

Checklist:

• [] My code follows the style guidelines of this project
• [] I have performed a self-review of my code
• [] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [] My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• [] New and existing unit tests pass locally with my changes
• [] Any dependent changes have been merged and published in downstream modules

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.1% Duplication on New Code

See analysis details on SonarCloud

[dustinxie]

see comments in 4312, better to also set method and cr.op and use common unpack code after switch