feat: remove specific value definition for keyring trace flags #215

[mattsb42-aws]

Issue #, if available:
resolves: #215

Description of changes:

Addresses the issues discussed in #215.

I went with a custom class rather than just object() because it turns out that copy.deepcopy cannot copy bare instances of object. attrs also gives us a nicer repr value.

>>> class Foo(Enum):
...     @attr.s
...     class FooValue(object):
...             name = attr.ib()
...     BAR = object()
...     BAZ = FooValue("BAZ")
... 
>>> Foo.BAR
<Foo.BAR: <object object at 0x10ceefc50>>
>>> Foo.BAZ
<Foo.BAZ: Foo.FooValue(name='BAZ')>

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[WesleyRosenblum]

Any reason these need to be appreviated?

[mattsb42-aws]

See reason for WRAPPING_KEY_ prefix.

[WesleyRosenblum]

Do these need to all start with "WRAPPING_KEY"?

[mattsb42-aws]

I think we went with that because that's what the prevailing naming was at the time (last summer). It looks like the names in the spec are now the simpler version. I'll update to use those names instead.

[WesleyRosenblum]

You could put a comment for each of these, copying the description from the spec, like https://github.com/aws/aws-encryption-sdk-java/blob/930793314224381e37a1331b685069a0bd55e6aa/src/main/java/com/amazonaws/encryptionsdk/keyrings/KeyringTraceFlag.java#L33