Introduce dedicated types for more Lexicon string formats

[str4d]

• datetime
• nsid
• language

[str4d]

There are three remaining Lexicon string formats that don't have dedicated types:

• at-uri
  • I add a RecordKey type in this PR, but after reading the AT URI Scheme spec again I'm no longer sure whether at-uri is specifically the Restricted AT URI Syntax (which I can implement with regex), or if it needs to be parsed as the Full AT URI Syntax (which is a bunch of extra work, similar to uri).
• cid
  • This is a CID in string format. This should be straightforward, but I haven't implemented it because the cid crate is currently behind the dag-cbor feature flag, so I want to check what your intentions are here before implementing anything. If you want this to remain configurable, then we'd need a fallback Cid(String) implementation (and would not be validating CIDs during parsing in that case).
• uri
  • This definitely cannot be implemented with the url crate, and requires a bunch of extra work.

[sugyan]

Thanks a lot!
I'm not sure about at-uri. Looking at the original atproto repository, I see that both ensureValidAtUri and ensureValidAtUriRegex, but it seems that only ensureValidAtUri is actually used.

I see, we need to use cid crate to validate the cid string format. Currently, I only add it to dependencies when the dag-cbor feature is enabled, but if it is needed for validation, you can change it so that it is added regardless of the feature.

In the future, it might be nice to add a feature like no-validation and thereby reduce the number of crate dependencies that are used only for validation.