refactor(afs)!: decouple anti-fee-sniping from create_psbt

[aagbotemi]

Summary

Decouples BIP-326 anti-fee-sniping from Selection::create_psbt, addressing
post-merge review on #5 :

• feat: enable random anti-fee sniping #5 (review)

Changelog notice

Removed:

• PsbtParams::enable_anti_fee_sniping
• Selection::create_psbt_with_rng
• CreatePsbtError::InvalidLockTime
• CreatePsbtError::UnsupportedVersion

Added:

• pub fn apply_anti_fee_sniping(...) — now part of the public API at
  the crate root.
• pub enum AntiFeeSnipingError { UnsupportedVersion(Version) } — the
  single failure mode AFS can produce.
• Selection::apply_anti_fee_sniping_with_rng(&self, psbt, tip_height, rng)
• Selection::apply_anti_fee_sniping(&self, psbt, tip_height)

Rename:

• utils.rs renamed to anti_fee_sniping.rs for discoverability.

Checklists

• I followed the contribution guidelines
• I've added tests for the new feature
• I've added docs for the new feature

[evanlinjin]

Thanks for working on this. Due to #57, I propose that we just remove the complex PsbtParams field relationships in this PR and leave moving AFS out in a separate PR.

struct PsbtParams {
    /// Documentation explaining:
    /// * `None` means no AFS.
    /// * Contained is the tip height.
    anti_fee_sniping: Option<absolute::Height>,
}

[aagbotemi]

Thanks for the review. I'll update the PR

[aagbotemi]

Closed in favor of #65