Estimate gas price API Endpoint

[MitchTurner]

Closes: #1648

[bvrooman]

Would it make more sense to accept block_horizon as an instance of BlockHorizon here, rather than converting in the function itself? Or maybe some T that can .into() into a BlockHorizon?

[MitchTurner]

I don't have a strong opinion either way, so I went with the precedence that had already been set, for example with client's endpoint block_by_height

pub async fn block_by_height(&self, height: u32) -> io::Result<Option<types::Block>> {
        let query = schema::block::BlockByHeightQuery::build(BlockByHeightArgs {
            height: Some(U32(height)),
        });

        let block = self.query(query).await?.block.map(Into::into);

        Ok(block)
    }