Specific timeouts per validator duties http request

It's now possible to configure a timeout per request, and we might wanna reduce timeouts for some requests as there is generally no value it keeping request open after certain time as it would be too late anyways.

Right now, all http requests have a timeout of `SECONDS_PER_SLOT` (12s on mainnet) which is globally configured
https://github.com/ChainSafe/lodestar/blob/8cb08ff56f66022c84d8ca1a91e82dba290e644a/packages/validator/src/validator.ts#L171-L173

More strict timeouts can improve fallback behavior as well, Lighthouse already uses this approach, see [validator_client/src/lib.rs#L74-L86](https://github.com/sigp/lighthouse/blob/f1d88ba4b1256e084d3a2a547e0ce574b896246c/validator_client/src/lib.rs#L74-L86).

Related https://github.com/sigp/lighthouse/issues/2313