Add ability to prepare the next operation and it's associated nonce for OpeningKey and SealingKey

[skmcgrail]

Issues:

Addresses #570

Description of changes:

Provides the ability for a user to precompute the next operation's nonce value for OpeningKey and SealingKey types via OpeningKey::prepare_operation and SealingKey::prepare_operation which return OpeningKeyOpMut and SealingKeyOpMut respectively containing the nonce for the encapsulated operation. Each type provides a nonce() method by which the nonce can be retrieved by the caller prior to invoking each types respective opening and sealing operation which will use the nonce.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.80%. Comparing base (c358484) to head (5c56524).
Report is 126 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
- Coverage   95.80%   92.80%   -3.00%     
==========================================
  Files          61       67       +6     
  Lines        8143     9858    +1715     
  Branches        0     9858    +9858     
==========================================
+ Hits         7801     9149    +1348     
- Misses        342      426      +84     
- Partials        0      283     +283     

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