Add ExplorerUrl to replace Network explorer methods

[cycraig]

Description of change

Separates explorer URL code out of Network into its own ExplorerUrl struct. This is primarily to avoid the explorer being unset on private Tangle networks unexpectedly when retrieved from e.g. an IotaDID.

Also corrects the examples to print both the explorer URL for messages and identity-resolver URL for DIDs, including the Wasm examples.

This fixes examples like create_did which print the wrong URL for messages.

E.g.

DID Document Transaction > https://explorer.iota.org/mainnet/identity-resolver/message/<message_id>

should be

DID Document Transaction > https://explorer.iota.org/mainnet/message/<message_id>

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Unit tests and Wasm examples pass locally.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cycraig]

Ideally we would move the explorer stuff out of Network completely, since it's really only for the examples.

[PhilippGackstatter]

Looks good for fixing the examples. I would also really like to move out the urls. Maybe we can have static instances, along the lines of:

lazy_static!{
    static ref EXPLORER_MAIN: ExplorerUrl = ExplorerUrl::new(Url::parse("https://explorer.iota.org/mainnet/identity-resolver").unwrap());
    static ref EXPLORER_DEV: ExplorerUrl = ExplorerUrl::new(Url::parse("https://explorer.iota.org/devnet/identity-resolver").unwrap());
}

impl ExplorerUrl {
    pub fn message_url(&self, message_id: &str) -> Result<Url> { ... }
    pub fn resolver_url(&self, did: &str) -> Result<Url> { ... }
}

Would be even nicer if these were infallible, since it's technically just formatting.

This can be done in a separate PR though, and this is good as-is.

[cycraig]

Thanks for the review and I definitely like the ExplorerUrl code. The only change I would make is to allow the creation of a custom ExplorerUrl for the private Tangle examples.

I'm going to leave that for a future PR as it's a nice-to-have improvement currently.

Edit: updated this PR to include ExplorerUrl.

[PhilippGackstatter]

Looks great! I really like the simplification of the Network type. Some minor comments.