Support ECDH for file encryption in public-key mode in an Acrobat-compatible way

[MatthiasValvekens]

Description of the changes

While the PDF standard is silent on this issue, Acrobat implements the dhSinglePass-stdDH-sha*kdf ephemeral-static ECDH family of schemes from RFC 5753, allowing documents to be encrypted for ECC key recipients. This PR implements that feature in pyHanko.

Caveats

Support for this mode of encryption in PDF readers & libraries is spotty at best, but that's not a huge deal, since presumably someone who's willing to advertise an ECC public key as a target for PDF encryption has the means to decrypt it.

Checklist

Please go over this checklist to increase the chances of your PR being worked on in a timely manner. Deviations are allowed with proper justification (see previous section).

• [x] I have read the project's CoC and contribution guidelines.
• [x] I understand and agree to the terms in the Developer Certificate of Origin as applied to this contribution.
• [x] All new code in this PR has full test coverage.

For new features (delete if not applicable)

• [x] I have discussed the implementation of this feature with the project maintainer(s) on the discussion forum or over email.
• [x] I have verified that my changes do not break existing API or CLI functionality, or ensured that all breaking changes are clearly documented in this PR.
• [x] All public API functionality in this PR is documented.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (e4e5cdd) 98.14% compared to head (f4a7365) 98.15%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #271 +/- ## ========================================== + Coverage 98.14% 98.15% +0.01% ========================================== Files 104 104 Lines 14569 14670 +101 ========================================== + Hits 14299 14400 +101 Misses 270 270 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `98.15% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/271?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens) | Coverage Δ | | |---|---|---| | [pyhanko/pdf\_utils/crypt/pubkey.py](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/271?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#diff-cHloYW5rby9wZGZfdXRpbHMvY3J5cHQvcHVia2V5LnB5) | `99.78% <100.00%> (+0.05%)` | :arrow_up: |

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