Introduce new field to BlockInfo structure

[shd]

Add 'intent' field to BlockInfo structure, as an enum with two variants: 'Validate' and 'Apply'.

This field is to be generated by mithril/upstream fetcher as 'Apply' (important for the ticket).
And in consensus module it should be generated as 'Validate' (may be easily left out of the ticket).

Later (outside the scope of this ticket) this field is to be checked by other modules, and maybe they would have different behaviour based on that (e.g. update internal structures or send validation messages).

Expected outcome: the project works like before with upstream.

It's not important to fully make correct PR, just have something compiling/working in github to allow work on other modules.

[sandtreader]

Must be two bool flags, because both can be true.

[shd]

Maybe, this kind of enum looks reasonable for this purpose:

#[bitmask(u8)]
#[derive(serde::Serialize, serde::Deserialize)]
pub enum Intention {
    Apply,
    Validate
}

Then one may specify Apply | Validate to combine values, and use intention.contains(Apply) to check.

[alexwoods]

Can be both be false? Because if not, an enum is safer

[shd]

Obviously not, sure.
I'd personally just abstract these functions as:
'is_apply()'
'is_validate()'
Since storage gives us so many questions.