Add changelog

Co-authored-by: Agustin Mista <agustin.mista@moduscreate.com>
Co-authored-by: Thomas BAGREL <thomas.bagrel@tweag.io>
Co-authored-by: Nicolas BACQUEY <nicolas.bacquey@tweag.io>

Author: 3 people

changelog.d/20260421_170000_agustin.mista_concrete_certs_and_votes.md

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+<!--
+A new scriv changelog fragment.
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+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Breaking
+
+- A bullet item for the Breaking category.
+
+-->
+
+### Non-Breaking
+
+- Introduce `Ouroboros.Consensus.Util.Bitmap` providing `ByteString`-based compact bitmaps.
+- Define `PerasBLSCrypto` scheme with support for all the voting committee superclasses.
+- Define concrete `PerasVote` and `PerasCert` types using BLS signatures.
+- Define `PerasVoteCompatibleWithVotingCommittee` and `PerasCertCompatibleWithVotingCommittee` type classes with conversions between concrete Peras types and their abstract voting committee counterparts.
+- Instantiate `VotingCommitteeSupportsPeras` for both `WFALS` and `EveryoneVotes`.
+
+<!--
+### Patch
+
+- A bullet item for the Patch category.
+
+-->