Config: `DynamicHasher`

Currently, Config params like Hasher and Signature are hardcoded to specific values that must match what's on the chain.

Could we make it possible to set config like:

impl Config {
    // ...
    type Hash = DynamicHash
    type Hasher = DynamicHasher;
}

Where DynamicHasher in this example would use the metadata to determine how the given chain wants to hash things (it would know about certain hashing functions, eg "blake2_256", so that if the metadata specifies a hasher that we know about, it's all good, and otherwise we'd throw an error when trying to hash or whatever).

This sort of thing may require changes to the metadata (and ultimately a V16 metadata release, which we don't want to jump to doing too quickly as V15 has just merged), but would allow us to have more flexible defaults for, in this example Hash and Hasherwhich would only need configuring if the chain used some custom hashing thing Subxt didn't know about.

Start by examining the Config and DynamicHasher concepts described in the issue, then review how metadata currently represents hashers in V15. Define how known hash functions are selected from metadata, how unknown hashers fail, and whether the design requires a V16 metadata change while preserving custom configuration for unsupported chains.