Preserve XSD dateTime and dateTimeStamp lexical representations.

[timothee-haudebourg]

Description

Adds a Lexical<T> wrapper type that preserves the lexical representation of xsd_types::DateTime and xsd_types::DateTimeStamp.

Also bumps json-ld to version 0.21.2, making sure that typed string literals are not canonicalized.

Other changes

Pinned Rust version to 1.81.0 in the CI, to avoid clippy warnings.

Tested

• Unit tests for Lexical<T>.
• Test suite.

[timothee-haudebourg]

I had to disable the vcdm_v2_sign::ecdsa_secp256r1_2020 test for unrelated reasons (it's using either the wrong cryptosuite or the wrong vcdm version, not sure why we didn't detect the error sooner).

[sbihel]

I'm approving to unblock other work, but the failure of ecdsa_secp256r1_2020 is worrying.

It looks like just adding https://w3id.org/security/v2 to the context fixes the test, but something larger seems to be at play. I would suggest uncommenting the test with this fix, and have follow-ups for:

• addressing the Clippy warnings from Rust 1.82
• investigating this context issue, and either have a fix or determine if it's not a big deal as it's an old cryptosuite