feat: Implement BTCHeaderBytes and BTCHeaderHashBytes types

[vitsalis]

After the discussion on #26 , we agreed that there is a need for custom types that wrap Header and Header Hash bytes objects so that we can have control over our input and output formats depending on the context (CLI vs. RPC etc).

The types implementations are put under a global directory (a practice also followed by cosmos) since they are probably going to be needed by other modules (e.g. btccheckpoint) and implement various marshalling and umarshalling operations, most notably from hex strings to bytes and vice versa.

[vitsalis]

Regarding Header Height: While having a type for it would be useful (instead of having uint64 everywhere), I believe that adding such custom type into our proto files will lead to unnecessary complexity. More specifically, having something like:

message Height {
    uint64 header_height = 1 [ (gogoproto.customtype) = "HeaderHeight"]
}

and in another file:

type HeaderHeight uint64

would lead to the Height.HeaderHeight being a reference to uint64, meaning that we would have difficulty performing arithmetic operations etc.

[aakoshh]

would lead to the Height.HeaderHeight being a reference to uint64, meaning that we would have difficulty performing arithmetic operations etc.

Would it? At least addition seems to work fine: https://go.dev/play/p/0FZkJKYjlnC

[aakoshh]

Can't it not be a repeated BTCHeaderHashBytes?

[vitsalis]

castrepeated expects a slice of the corresponding type. Changed this to customtype pointing to types.BTCHeaderHashBytes and works as expected.

[vitsalis]

Mainly used this because it is used extensively in Cosmos, but requires defining a new type.

[aakoshh]

Maybe it would make sense to have a dedicated package for these types, like decimal.go in the SDK, so you can do this in one line, like headerBytes, err := btc_header.NewFromHex(headerHex) and similarly for hashes.

[vitsalis]

For now I added these methods under the bbl namespace.

[aakoshh]

Smashing! I admit I got a bit lost in the many marshalling variants, and perhaps the ergonomics could be improved so a variable declaration doesn't need to be separate from assignment, but otherwise it's a great leap! 👨‍🚀

[vitsalis]

Thanks @aakoshh for the comments! Updates and answers have been posted.

[aakoshh]

Thanks for all the changes, looks good 👍