fix(slack): use attachments and add title and color fields

[piksel]

• Use legacy attachments due to the color support
• Use the normal text field as title
• Allow color and title to be set using query and params
• Don't override Bot name by default

[codecov]

Codecov Report

Merging #119 (f1b343b) into main (4034690) will increase coverage by 2.79%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   56.50%   59.30%   +2.79%     
==========================================
  Files          55       55              
  Lines        1168     1177       +9     
==========================================
+ Hits          660      698      +38     
+ Misses        464      429      -35     
- Partials       44       50       +6     

Impacted Files	Coverage Δ
pkg/services/slack/slack_token.go	88.46% <ø> (ø)
pkg/services/slack/slack.go	65.21% <66.66%> (+1.58%)	⬆️
pkg/services/slack/slack_config.go	92.59% <71.42%> (+19.86%)	⬆️
pkg/services/slack/slack_json.go	100.00% <100.00%> (ø)
pkg/services/smtp/smtp_authtype.go	0.00% <0.00%> (ø)
pkg/services/smtp/smtp.go	81.42% <0.00%> (+0.13%)	⬆️
pkg/services/smtp/smtp_encmethod.go	62.50% <0.00%> (+6.94%)	⬆️
pkg/services/pushover/pushover.go	67.74% <0.00%> (+67.74%)	⬆️

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[piksel]

Remove maxlength check

shoutrrr/pkg/services/slack/slack.go

Lines 40 to 42 in 38a2e57

	if len(message) > maxlength {
	return errors.New("message exceeds max length")
	}

since it's way lower than the actual slack limits, slack automatically truncates too long messages, and because it doesn't help the user in any way since this is post configuration/initialization.