feat: add date time message entity

[fluffur]

Parse new date time into message entity

func (h *Handler) Help(c *botapi.Context) error {
	log.Printf("%+v\n", c.Message().Entities)

	return nil
}

[
  {
    "type": "bot_command",
    "offset": 0,
    "length": 5
  },
  {
    "type": "date_time",
    "offset": 6,
    "length": 19,
    "unix_time": 1783148399
  }
]

To make sending messages with formatted time work in HTML and MD formats it is required to do some changes in gotd/td parsers as well

[codecov]

Codecov Report

❌ Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.46%. Comparing base (3466ee3) to head (6a131d2).

Files with missing lines	Patch %	Lines
entities.go	0.00%	18 Missing ⚠️
codec_misc.go	0.00%	14 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
- Coverage   78.78%   78.46%   -0.33%     
==========================================
  Files         118      118              
  Lines        8316     8350      +34     
==========================================
  Hits         6552     6552              
- Misses       1237     1269      +32     
- Partials      527      529       +2     

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