Use EcdsaSecp256k1RecoveryMethod2020 v2 context

[clehner]

• Add the signature suite's v2 context file under w3id.org, as updated in Update dependencies and context files decentralized-identity/EcdsaSecp256k1RecoverySignature2020#28.
• Update the test vectors from the EcdsaSecp256k1RecoverySignature2020 specification.
• Deprecate the v1 context and extra/delta context; use the v2 one instead.
• Only add context to proof if needed (i.e. if not already in the credential) - following the behavior of the other proof suite implementations.

Using the deprecate macro on the lazy_static pub static ref did not seem to work, so I wrote it as a docstring instead.

[sbihel]

Not sure I fully understand the deprecation tags, is it to warn people who are building VCs with ssi/DIDKit? Wouldn't it also generate warning for any didkit build?

[clehner]

The deprecation tags are to alert anyone who uses the old context constants from Rust directly, and also to provide useful information in the rustdocs documentation. This shouldn't produce any warning in builds (as I added the ignores where the context is used internally) and it doesn't add any warning when processing JSON-LD documents using the old context.
Producing a runtime warning when the deprecated context is used might be useful though... It's not really bad if someone is using the old contexts, just that one was missing some fields and the other was only on Spruce's domain'; the new one is more following latest best practices and hopefully will be used more by other implementations.