Add HasTextEnvelope instance for EraHistory

[palas]

Changelog

- description: |
    Added `HasTextEnvelope` instance for `EraHistory`
  type:
  - feature        # introduces a new feature
  - compatible     # the API has changed but is non-breaking

Context

This change in the API is required to support the related PR in cardano-cli: IntersectMBO/cardano-cli#1079

How to trust this PR

I would look at all the changes, which are few. The fact that it type-checks already gives a lot of guarantee. The approach is pretty similar to that of other HasTextEnvelope instances. The tests in the related cardano-cli PR can offer further assurance.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[Jimbo4350]

LGTM however where is this being used?

[palas]

LGTM however where is this being used?

@Jimbo4350 Here: https://github.com/IntersectMBO/cardano-cli/pull/1079/files#diff-4a005a66492bcc5f20c8736e734a3436aade94dfc6e8c96d8a59d22ad4c07c81R1788
And here: https://github.com/IntersectMBO/cardano-cli/pull/1079/files#diff-cd47229c21f74eac7d48b70c9fe6249cfa34d949e1dca5f2f8aa80be611d26a3R1878

[Jimbo4350]

LGTM 👍 . Could you add a small haddock just explaining where this is consumed.