[MM-20684] Slash Command Autocomplete (#14557)

* [MM-20684] Initial implementation of the Command Autocomplete (#13602)

* Implement Autocomplete Data

* Change CommandName to Trigger

* Fix Autocomplete test

* Make stylistic changes

* Rename a bunch of fields and methods

* Fix variable names, safer type assertions

* [MM-20684] plugin autocomplete implementation (#14259)

* Add an endpoint for command autocomplete suggestions

* Add full Suggestion to the AutocompleteSugestion struct

* Add Dynamic Argument support

* Tidy up things

* Fix missed test case

* Add support of the named arguments

* Update autocomplete API

Fix review issues

Implement dynamic args as a local request

* Fix ineffassign

* Add support of the uppercase letters in arguments

* Add support of the optional arguments

* Remove ineffectual assignment

* Add support for icons (#14489)

* Address couple of nits

* Add comment to IconData

* Add types to all consts

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

app/app_iface.go

@@ -190,6 +190,8 @@ type AppIface interface {
 	// GetSessionLengthInMillis returns the session length, in milliseconds,
 	// based on the type of session (Mobile, SSO, Web/LDAP).
 	GetSessionLengthInMillis(session *model.Session) int64
+	// GetSuggestions returns suggestions for user input.
+	GetSuggestions(commands []*model.Command, userInput, roleID string) []model.AutocompleteSuggestion
 	// GetTeamGroupUsers returns the users who are associated to the team via GroupTeams and GroupMembers.
 	GetTeamGroupUsers(teamID string) ([]*model.User, *model.AppError)
 	// GetTeamSchemeChannelRoles Checks if a team has an override scheme and returns the scheme channel role names or default channel role names.