Add support for enum constraints

[0xC0D3D00D]

This adds support for enum constraints in jsonschema. Supported constraints:

enum Enum {
  ENUM_UNSPECIFIED = 0;
  ENUM_VAL1 = 1;
  ENUM_VAL2 = 2;
  ENUM_VAL7 = 7;
}

message E {
  Enum const_enum = 1 [(buf.validate.field).enum.const = 2];
  Enum defined_only_enum = 2 [(buf.validate.field).enum.defined_only = true];
  Enum in_enum = 3 [(buf.validate.field).enum = { in: [1, 2]}];
  Enum not_in_enum = 4 [(buf.validate.field).enum = { not_in: [0, 7]}];
  # It's also possible to have a combination of these constraints
  Enum defined_only_not_in_enum = 5 [(buf.validate.field).enum = {defined_only: true, not_in: [0]}];
}

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[0xC0D3D00D]

@Alfus @mfridman, Are there any chances for this to get reviewed in upcoming weeks?