Add Runtime API and dynamic examples, and util methods to StorageKeyPart

[jsdw]

The dynamic example shows how to use the subxt::dynamic::* functions to interact with storage, constants, runtime APIs and transacitons. This is the sort of usage that's super common when accessing historic blocks etc.

The runtime_apis example shows how to call a couple of Runtime APIs.

I added a couple of helpers on the StorageKeyPart struct:

• key_part.hash_bytes(): return the bytes for the hash
• key_part.value_bytes(): return the bytes for the value.

It was already possible to decode the value, and so it made ense to be able to access the bytes for the value too (for instance if you want to codec::Decode them into some type rather than DecodeAsType them.