feat: support proxying to specific host via parameters

[zghong]

Description

Support to select specific host via replica_index and node_index (alias shard_index) parameters when proxying requests.

• index starts from 1, 0 means no specific host index.
• shard_index is alias for node_index, and override node_index if both are specified.

Closes #523

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

• Linter passes correctly
• Add tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary

Does this introduce a breaking change?

• Yes
• No

Further comments

No.