chore: fill in gaps in testing

[kessplas]

Issue #, if available: #142, #166, #151

Description of changes:
Fills in gaps in testing. Notably:

• adds integ tests for key commitment both valid and invalid configurations
• adds tests for encryption BytesIO objects
• adds more tests around invalid instruction file cases
• adds MRK tests
• splits coverage between unit and integ tests
• adds custom CMM/keyring tests
• adds tests for error propagation from S3/KMS

There are exactly two functional changes in this PR:

• add a getattr method which proxies requests to the wrapped S3 client. this lets customers call "normal" S3EC operations on the S3EC instance
• add validation against invalid (non-US-ASCII) characters in encryption context.

You can view the test plan / its evaluation in this dummy PR: #171

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