fix: revert "Eth API: drop support for 'pending' block parameter."

[raulk]

This reverts commit 9412753.

Related Issues

Some tools require pending support on some RPCs, concretely eth_getTransactionCount. This backs out the patch that generally dropped such support, in case we want to restore master to working condition while #10462 is being reviewed.

Proposed Changes

Re-introduce pending block support.

Additional Info

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