Add hashing to secp256k1

[skaunov]

This PR is draft since I need a hint! When hashing the lib checks that the isogenous generator maps correctly onto the target curve generator. The problem is I couldn't find the relevant generator for RFC9380 yet (via a reference, or computationally). I found out that there's a path via the isogeny kernel, but I wonder if somebody could suggest a simpler way to obtain that to continue with this.

Description

Adds hashing to the <curves/secp256k1>.

closes: none - just a development

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master): applicable, but I guess it will go smoother when v0.5 will be merged into master
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work. The design of this development is very straightforward,
• Wrote unit tests: partly
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md: feels like a bad idea until v0.5 is merged into master
• Re-reviewed Files changed in the GitHub PR explorer