Improve binding logic

[RobertoPrevato]

• Improve the code responsible of mapping input request parameters into
  instances of desired types. Change the inner workings of
  blacksheep.server.bindings to make the code more configurable and easier to
  maintain.
• Add support for StrEnum and IntEnum to binders for request handlers'
  parameters. See #588.
  Enums can be mapped by key and by value. The class that matches StrEnum
  make case sensitive checks; override the __missing__ method of your
  user-defined enums to support case insensitive checks; or define a custom
  StrEnumConverter class.
• Add support for Literal to binders for request handlers'
  parameters. See #588.
  String literals are case sensitive by default.
• Minor breaking change. Remove the dependency on python-dateutil
  (#544), which was always
  used to parse input datetime parameters. The datetime parsing logic is
  replaced with a function that only supports the most common ISO formats:
  %Y-%m-%dT%H:%M:%S.%f, %Y-%m-%dT%H:%M:%S, %Y-%m-%d, and is much more
  performant for such formats. The new code API offers a simple way to keep
  using python-dateutil for those who desire doing so.
• Fix erroneous assumption when parsing a request body declared as bytes.
  When the declared requested input body is bytes, the framework has been
  corrected to not require URL-safe base64 encoded data, and to read the input
  body as-is.
• Add support for defining convert functions in custom BoundValue classes
  that are used to convert Python objects from parsed JSON into more specific
  classes.
• Correct bug that prevented request body input to be mapped properly to
  a list using the default logic.