feat(services): add tag-based query/params helper

[piksel]

This adds a format.PropKeyResolver helper that services can use to automatically implement the query/params serialization based on the config key tags.
Example:

import (
        "github.com/containrrr/shoutrrr/pkg/format"
	"github.com/containrrr/shoutrrr/pkg/types"
)

type Config struct {
	Title     string   `key:"subject" default:"Shoutrrr Notification"`
}

// Public version of GetURL that creates a new PropKeyResolver when accessed from outside the package
func (config *Config) GetURL() *url.URL {
	resolver := format.NewPropKeyResolver(config)
	return config.getURL(&resolver)
}

// Private version of GetURL that can use an instance of PropKeyResolver instead of rebuilding it's model from reflection
func (config *Config) getURL(resolver types.ConfigQueryResolver) *url.URL {
	return &url.URL{
		Host:       "example",
		Scheme:     Scheme,
                // format.BuildQuery now requires a different interface,
                // which could either be a PropKeyResolver or a Config that implements the old ServiceConfig interface
		RawQuery:   format.BuildQuery(resolver),
	}
}

func (service *Service) Send(message string, params *types.Params) error {
        // Create a copy of the config this send
	config := service.config

        // Mutate the config with overrides from params
	if err := service.propKeyResolver.UpdateConfigFromParams(&config, params); err != nil {
		return err
	} else {
		return service.doSend(client, message, &config)
	}
}

Almost all services that uses query fields has been upgraded to use PropKeyResolver, and should automatically override the configuration keys when props are passed to the Send method.
This also means that services config no longer need to implement the Get(), Set() and QueryFields methods, so those along with standard.QuerylessConfig has been removed.

Fixes #78

[codecov-io]

Codecov Report

Merging #92 (fd12d74) into master (2d29e9b) will decrease coverage by 5.17%.
The diff coverage is 55.20%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   56.57%   51.40%   -5.18%     
==========================================
  Files          52       52              
  Lines        1278     1142     -136     
==========================================
- Hits          723      587     -136     
+ Misses        526      524       -2     
- Partials       29       31       +2     

Impacted Files	Coverage Δ
pkg/services/discord/discord_config.go	63.15% <ø> (ø)
pkg/services/gotify/gotify_config.go	0.00% <ø> (ø)
pkg/services/hangouts/hangouts_config.go	100.00% <ø> (ø)
pkg/services/join/join.go	0.00% <0.00%> (ø)
pkg/services/mattermost/mattermost_config.go	58.82% <ø> (ø)
pkg/services/pushbullet/pushbullet_config.go	54.54% <ø> (ø)
pkg/services/pushover/pushover.go	0.00% <0.00%> (ø)
pkg/services/rocketchat/rocketchat_config.go	69.23% <ø> (ø)
pkg/services/slack/slack.go	22.22% <0.00%> (+4.83%)	⬆️
pkg/services/standard/standard_failures.go	0.00% <0.00%> (ø)
... and 20 more

---

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[atighineanu]

I've built from this PR. Indeed it doesn't show ":" preceeded by empty fields, but there is still a crash after token input.

[piksel]

@atighineanu I guess it's because of array index out of range?
The token needs to be entered as a comma-separated 3-part array ( aaaaaaaaaaaaa,bbbbbbbbbbbb,cccccccccccc ), there is nothing that tells the user that, but now at least it's possible to enter it.
I am looking into either using a custom Generator for Slack or make it possible to specify a minimum length for array fields so that the Basic generator handles them instead of crashing...