Add a plugin configuration setting for the percentage of users to use as canaries

Description

Add a configuration setting to the plugin for setting the percentage of users to send to the canary build.

The configuration for a plugin needs to be added in two places:

1) In `server/configuration.go` as part of the configuration struct
2) In `plugin.json` under "settings_schema" (this will add it to the system console)
see for more details https://developers.mattermost.com/extend/plugins/manifest-reference/

QA Test Steps

No testing required.

Reporter

None

Status

Mana

4

Assignee

Stylianos Rigas

QA Assignee

None

Reporter

Joram Wilander

Epic Link

Fix versions

None

Mattermost Team

Cloud

Sprint

None

Labels

None

QA Testing Areas

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GitHub Issue

None

Components

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Configure