[BUG] Missing toString() method in many requests classes

[guillaume-alvarez]

Describe the bug

Whereas SearchRequest has a toString() method, making debugging much easier, it is not present in all other requests.

For instance it is missing in

• MultiSearchRequest
• CreateIndexRequest
• MultiGetRequest

Related component

Clients

To Reproduce

1. Create a MultiSearchRequest object
2. Try to log it
3. See something like org.opensearch.action.search.MultiSearchRequest@3eb87ae4

Expected behavior

1. Create a MultiSearchRequest object
2. Try to log it
3. See something like {requests=[SearchRequest{...},...]}

Additional Details

Plugins
N/A

Screenshots
N/A

Host/Environment (please complete the following information):

• any Java client

Additional context
I will try to provide a fix in PR.