DynSigner struct: for Solana CLI usage to wrap Box<dyn Signer> to align with Client<C>

[ochaloup]

When using Solana labs CLI utilities (solana-clap-utils, https://github.com/solana-labs/solana/tree/master/clap-utils -> https://github.com/solana-labs/solana/blob/6b013f46ebd30c82f7fa9c50c5a0e9ae32df3c44/clap-utils/src/keypair.rs#L357) the loaded signer of type Box<dyn Signer> is not aligned to the expected Client<C> type of <C: Clone + Deref<Target = impl Signer>>. Using the dyn Signer fails with error:

the size for values of type `dyn solana_sdk::signature::Signer` cannot be known at compilation time [E0277] doesn't have a size known at compile-time Help: the trait `Sized` is not implemented for `dyn solana_sdk::signature::Signer` Note: required by a bound in `anchor_client::Client::<C>::new_with_options`

Adding the helper DynSigner makes possible to match those types and use the Signer loded from the Solana utils with the Anchor client.

let fee_payer: Arc<dyn Signer> = ...
let anchor_client: Client<Arc<DynSigner>> = Client::new_with_options(
  anchor_cluster,
  Arc::new(DynSigner(fee_payer.clone())),
    CommitmentConfig::confirmed(),
);

[vercel]

@ochaloup is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.

[acheroncrypto]

Not sure how much usage will come from solana-clap-utils instead of regular solana-sdk's read_keypair_file and it looks like people are already able to do this themselves(without modifications Anchor).

That being said since the code is very small, I don't see any harm with including this in anchor-client. Could you add an entry to the CHANGELOG?

[ochaloup]

@acheroncrypto yes, sure, thank you. I added a description into the CHANGELOG.