Implement crude server validation

[adwsingh]

Issue #, if available:

Description of changes:

This is a basic and crude server validation that is meant to be a stopgap till #546 is merged.

Tested it manually,

❯ curl -i -X POST http://localhost:8080/add-beer -H 'Content-Type: application/json' -d '{}'
HTTP/1.1 400 Bad Request
x-amzn-errortype: smithy.framework#MalformedRequestException
content-length: 46
content-type: application/json

{"message":"Missing required members: [beer]"}%
❯ curl -i -X POST http://localhost:8080/add-beer -H 'Content-Type: application/json' -d '{"beer":{"name":"a"}}'
HTTP/1.1 400 Bad Request
x-amzn-errortype: smithy.framework#ValidationException
x-amzn-errortype: smithy.framework#ValidationException
content-length: 123
content-type: application/json

{"message":"1 validation error(s) detected.Value with length 1 must have length greater than or equal to 3 at /beer/name;"}%

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