Fix AttributeValue marshaling and names in expressions

[jasdel]

Updates the attributevalue and expression package's handling of
AttributeValue marshaling fixing several bugs in the packages.

• Fixes Inconsistent struct field name marshaled. Fields will now
  be consistent with the EncoderOptions or DecoderOptions the Go struct
  was used with. Previously the Go struct fields would be cached with
  the first options used for the type. Causes subsequent usages to have
  the wrong field names if the encoding options used different TagKeys.

  • Fixes feature/dynamodb/attributevalue: Inconsistent struct field name marshaled #1569

• Fixes Support more than string types for map keys. Updates
  (un)marshaler to support number, bool, and types that implement
  encoding.Text(Un)Marshaler interfaces.

  • Fixes dynamodb/attributevalue: Support Text(Un)marshaler interfaces to avoid using special marshalers in simple cases #645
  • Fixes dynamodb/attributevalue: Support more than string types for map keys. #411

• Fixes Support for expression Names with literal dots in name. Adds new
  function NameNoDotSplit to expression package. This function allows
  you to provide a literal expression Name containing dots. Also adds a
  new method to NameBuilder, AppendName, for joining multiple name path
  components together. Helpful for joining names with literal dots with
  subsequent object path fields.

  • Related to dynamodb: Guidance for map keys with dots in dynamodb.expression aws-sdk-go#2570

• Fixes bug with AttributeValue marshaler struct tag usage that caused
  encoder option's TagKey to be ignored if the member also had a struct tag with
  dynamodbav. Now both tags will be read as documented, with the TagKey
  struct tag options taking precedence.