Add Arc testsnet

[AlexanderTar]

Adding Circle's Arc testnet chain

[zerosnacks]

Hi @AlexanderTar , thanks for your PR

Make sure to follow all these steps:

chains/src/named.rs

Lines 7 to 20 in 51b1911

	// When adding a new chain:
	// 1. add new variant to the NamedChain enum;
	// 2. add extra information in the last `impl` block (explorer URLs, block time) when applicable;
	// 3. (optional) add aliases:
	// - Strum (in kebab-case): `#[strum(to_string = "<main>", serialize = "<aliasX>", ...)]`
	// `to_string = "<main>"` must be present and will be used in `Display`, `Serialize`
	// and `FromStr`, while `serialize = "<aliasX>"` will be appended to `FromStr`.
	// More info: <https://docs.rs/strum/latest/strum/additional_attributes/index.html#attributes-on-variants>
	// - Serde (in snake_case): `#[cfg_attr(feature = "serde", serde(alias = "<aliasX>", ...))]`
	// Aliases are appended to the `Deserialize` implementation.
	// More info: <https://serde.rs/variant-attrs.html>
	// - Add a test at the bottom of the file
	// 4. run `cargo test --all-features` to update the JSON bindings and schema.
	// 5. run `cargo +nightly fmt --all` to properly format the code.

[twi-alexandertar]

Hey @zerosnacks - all done, sorry for not reading the instructions fully

[mattsse]

ty!