Prepare sensitive_url for crates.io

[macladson]

Issue Addressed

Another good candidate for publishing separately from Lighthouse is sensitive_url as it's a general utility crate and not related to Ethereum. This PR prepares it to be spun out into its own crate.

Proposed Changes

I've made the full field on SensitiveUrl private and instead provided an explicit getter called .expose_full(). It's a bit ugly for the diff but I prefer the explicit nature of the getter.
I've also added some extra tests and doc strings along with feature gating Serialize and Deserialize implementations behind the serde feature.

Additional Info

I also removed AsRef<str> as this was previously used quite awkwardly and instead I've implemented a .redacted() getter which I think is cleaner.

[mergify]

Some required checks have failed. Could you please take a look @macladson? 🙏

[michaelsproul]

Nice! Keen!

[mergify]

This pull request has been removed from the queue for the following reason: checks failed.

The merge conditions cannot be satisfied due to failing checks:

• ☑️ local-testnet-success

You can check the last failing draft PR here: #8359.

You may have to fix your CI before adding the pull request to the queue again.
If you update this pull request, to fix the CI, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.

[michaelsproul]

@mergify requeue

[mergify]

requeue

✅ The queue state of this pull request has been cleaned. It can be re-embarked automatically