add get_multiple_accounts to BenchTpsClient

[KirillLykov]

Problem

It turned out that requesting durable nonce blockhashes one by one is too expensive.
By requesting 100 of them (maximum possible), tps is x3 higher.

These changes will be used for #27151
In particular, at this line:

solana/bench-tps/src/bench.rs

Line 549 in 19de07a

match client.get_multiple_accounts(&nonce_pubkeys) {

Summary of Changes

• add get_multiple_accounts to BenchTpsClient`
• note, that this method is not implemented for BankClient because neither SyncClient nor AsyncClient doesn't have it

[CriesofCarrots]

wfm