fix: enable alloc feature on hex crate

[yanCode]

Description

I detected this issue while i was trying to run unit test under an individual crate bytes-hex.
I mean to execute cargo build or cargo test right under ./crates/bytes-hex. well, as bytes-hex is a crate, it's supposed to compile sucessfully & do unit tests independently. but it cannot even compile.
with an error like:

296:8
    |
296 | pub fn decode<T: AsRef<[u8]>>(data: T) -> Result<Vec<u8>, FromHexError> {
    |        ^^^^^^
note: the item is gated behind the `alloc` feature

but I also realized the unit tests working well in github Actions. then i forked the code and test a few more scenarios, for examples, if you modify the Cargo.toml in the root directory, changing
members = ["crates/*"]
into
members = ["crates/bytes-hex"]
then the github actions will fail with the exact same error, here is the github action log from my forked repo FYI.
however, as long as i include some crates which including bytes-hex as dependency, like
members = ["crates/model", "crates/bytes-hex"]
It works successfully as the default which including all crates. I mean members = ["crates/*"]

To this point, i realised that these crates like model much have smuggled the feature into bytes-hex somehow.
I did a lot of research, in the official doc of rust, i learned that is a concept Feature unification. In short, in a workplace if multi crates/dependencies refer to the same crate, as long as these refers can be resolved to the same version, it will make an union of features from each reference. and the union of features will be available globally among the whole workplace.

for example, web3 crate is commonly refered as a dependency in many crates, including model. if you step into the dependency of web3 crate . you can see hex is included with default features, that is it comes with alloc feature. because of this the feature alloc is propogating to bytes-hex. that's why it works if putting bytes-hex with other crates, it works fine.
(N.B: hex is a commonly used crate with default feature in many dependencies from the crates in the workplace, you can using cargo tree --all-features to check.)

Changes

As long as i can confirm the feature alloc of hex crate will be introduced by other dependencies into the workplace. so just update Cargo.toml inside bytes-hex. this can ensure tasks like build, unit test work inside bytes-hex and in the workplace level. while introducing very less (or even no) impacts to the whole workplace.

#How to test
now it can be tested freely either inside bytes-hex or in the workplace. in .devcontainer or in github actions.

Why open this PR

I open this PR because this is the first time I learnt how feature unification in rust exactly works although it claimed a few hours of mine with which I planed to learn some CoW knowledge.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[yanCode]

I have read the CLA Document and I hereby sign the CLA

[MartinquaXD]

Thanks for your contribution and the very detailed PR description. 👏