[staking] refactor statereader to support multiple contract indexers

[envestcc]

Description

This refactoring will simplify the integration work of the coming new staking contract indexer, with the main changes including:

• removing CandidateVotes() from the indexer interface, with vote counting implementation move to the staking protocol
• adding ContractAddress() in the indexer interface
• introducing []indexers in statereader, and implementation of various read interfaces based on it.

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• Code refactor or improvement

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 failed

Failed conditions
4.3% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud