Compile-time validation on builders about algonaut

[ivanschuetz]

Bringing up here again that this:

The current builders are minimal implementations of the pattern that do not apply any compile-time validation

is outdated. Our transaction builders were refactored to check for mandatory fields at compile time. The only remaining builders with the issue should be the algod/kmd/indexer builders, which have a total of 1 or 2 mandatory fields. Here we might need possibly just a factory (initializer that returns an error if e.g. the URL is incorrectly formatted) instead of the builder pattern.

[AlterionX]

A slight suggestion: I think the default builder should just be a tight wrapper of the custom endpoint builder struct. Reasoning being -- creating a custom endpoint should be a strictly more powerful ability than creating the default endpoint. It might also help with the discovery of the custom endpoint builder.

[ivanschuetz]

What about something like this:

#[derive(Default)]
pub struct AlgodBuilder {}

impl AlgodBuilder {
    pub fn with_token(url: &str, token: &str) -> Result<Algod, AlgonautError> {
        Self::with_headers(
            url,
            vec![("X-Algo-API-Token", &ApiToken::parse(token)?.to_string())],
        )
    }

    pub fn with_headers(url: &str, headers: Headers) -> Result<Algod, AlgonautError> {
        Ok(Algod::new(Client::new(url, headers)?))
    }
}

We could also remove AlgodBuilder and just add a convenience initializers to Algod.

The later would also simplify managing multiple versions - if there are new versions, we don't need to create multiple entry points / builders.

If the initialization becomes more complex later (unlikely?) we can consider re-adding the builder pattern. Currently it seems not justified.