Support `new_primary` in `/primary/evict` endpoint

[bufferflies]

Enhancement Task

Currently the /primary/evict endpoint transfers keyspace group primaries to a
random other member of each group. This enhancement adds an optional
new_primary field in the request body, allowing the caller to specify a target
node for the evicted primaries, consistent with the existing /primary/transfer
endpoint.

• Add new_primary (optional) body parameter to /primary/evict
• Pass the specified target to TransferPrimary instead of empty string
• Update Swagger annotation
• Add test coverage in TestEvictPrimary