fix(ws): use spec "showRewards" key in blockSubscribe params

[ozpool]

Closes #208.

Bug

BlockSubscribe and ParsedBlockSubscribe build the params object with the wrong field name:

if opts.Rewards != nil {
    obj["rewards"] = opts.Rewards
}

The Solana websocket spec for blockSubscribe calls this field showRewards, not rewards. The validator silently ignores the unknown rewards key, which means setting BlockSubscribeOpts.Rewards = &false has no effect today — rewards are always populated on the wire. The original report in #208 (open since 2024-05) cites the spec and notes there is no workaround.

Note: this is not the same as the HTTP getBlock RPC, where rewards is the correct field name per its own spec — only the websocket variant differs. rpc/getBlock.go is left untouched.

Fix

• rpc/ws/blockSubscribe.go: emit showRewards instead of rewards.
• rpc/ws/parsedBlockSubscribe.go: same fix in the parsed variant (PR Subscribe block #203 copied the buggy name).

Tests

rpc/ws/blockSubscribe_rewards_test.go (new), using the in-tree mockWSServer to intercept the subscribe request and assert the params shape:

• TestBlockSubscribeUsesShowRewardsField — pins that BlockSubscribe emits showRewards: false (and never rewards) when Rewards = &false.
• TestParsedBlockSubscribeUsesShowRewardsField — same for the parsed path with Rewards = &true.

$ go test -count=1 ./rpc/ws/... ./rpc/
ok   github.com/gagliardetto/solana-go/rpc/ws   0.313s
ok   github.com/gagliardetto/solana-go/rpc      0.596s

Diff size

 rpc/ws/blockSubscribe.go               | +1 -1
 rpc/ws/parsedBlockSubscribe.go         | +1 -1
 rpc/ws/blockSubscribe_rewards_test.go  | +107   (new)