Add support for custom default values

[mtdowling]

This commit updates Smithy IDL v2 to support custom default values
rather than only default zero values.

We didn't previoulsy support custom default values because we wanted to
be able to change default values if ever needed, we wanted to avoid
information disclosure of unreleased internal members, and we wanted
clients to be able to fill in a default zero value if a required member
is missing on the wire (implying that the member transitioned from
required to default).

While default zero values are a big simplification for clients, they do
come with awkward tradeoffs. For example, the zero value of an enum was
"" (or it could have been the first enum in the shape definition but
that's problematic due to model filtering, and it requires a lot of
up-front planning from service teams). We also observed that there are
likely 1,000+ members already used in AWS that have default values that
are only captured through documentation. If we could actually model
defaults, it would be easier to catch changes to default values in
automated diff tools and discuss them in API reviews.

To address these concerns and support custom default values, we
recommend the following:

• clients implement presence tracking and only serialize default values if
  the member is explicitly set to the default value. This allows services
  to change defaults if ever necessary.
• servers always serialize default values unless the member is marked with
  the @internal trait. This makes it explicit as to what the server
  thinks the default value is while still preventing unintended
  information disclosure of unreleased features.
• To avoid downtime and account for misconfigured servers that do not
  serialize required or default members, clients attempt to populate the
  member with a default zero value.

More details and specifics can be found in
defaults-and-model-evolution.md.

Enum changes:

• Given the addition of syntax sugar for assigning values to
  defaulted structure members, this commit also allows syntactic sugar for
  assigning values to enum and intEnum shapes without the use of the
  @enumValue trait.
• Now that there is no default zero value for enums, the @enumDefault
  trait has been removed.

Issue #, if available:

Description of changes:

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