feat: Enable retrieval of constants from runtime metadata

[emostov]

Closes #205

Example of new public api usage and outputs

    let client = ClientBuilder::<KusamaRuntime>::new().build().await?;
    let block_hash_count_meta = client.metadata()
        .module("System")?
        .constant("BlockHashCount")?;

    println!("name: {:#?}\n value: {:#?}\n ty: {:#?}\n documentation: {:#?}\n",
        block_hash_count_meta.name(),
        block_hash_count_meta.value::<u32>()?,
        block_hash_count_meta.ty(),
        block_hash_count_meta.documentation()
    );

  // name: "BlockHashCount"
  // value: 2400
  // ty: "T::BlockNumber"
  // documentation: [
  //    " The maximum number of blocks to allow in mortal eras.",
  // ]

[niklasad1]

nice, LGTM

[ascjones]

Suggested change

	key: &'static str,
	key: &str,

No need to restrict it to 'static

[ascjones]

I see now some of the other similar methods are also 'static, but I don't think they need to be if we make the respective Error variants have a String instead.

[ascjones]

LGTM

We can figure out whether to allow non 'static keys in another PR.