Add `IncomingViewingKey::prepare` convenience method.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.88%. Comparing base (33474bd) to head (c6ff554).

Files	Patch %	Lines
src/keys.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #424 +/- ## ========================================== - Coverage 79.93% 79.88% -0.06% ========================================== Files 30 30 Lines 3194 3196 +2 ========================================== Hits 2553 2553 - Misses 641 643 +2 ```

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