[feat] Notice.txt generation configuration

We created a tool to automatically generate NOTICE.txt from project folders. And there is a scheduled pipeline configured to run at every Monday which creates a PR if there is a change.
We selected noticegen directory to store the configuration files.

Tool repo: https://github.com/mattermost/notice-file-generator

Ticket: https://mattermost.atlassian.net/browse/CLD-3697
Signed-off-by: Akis Maziotis <akis.maziotis@mattermost.com>

noticegen/Readme.md

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+# Notice.txt File Configuration
+
+We are automatically generating Notice.txt by using first-level dependencies of the project. The related pipeline uses `config.yaml` stored in this folder.
+
+
+## Configuration
+
+Sample:
+
+```
+title: "Mattermost Playbooks"
+copyright: "©2015-present Mattermost, Inc.  All Rights Reserved.  See LICENSE for license information."
+description: "This document includes a list of open source components used in Mattermost Playbooks, including those that have been modified."
+search:
+  - "go.mod"
+  - "client/go.mod"
+dependencies: []
+devDependencies: []
+```
+
+| Field | Type   | Purpose |
+| :--   | :--    | :--     |
+| title | string | Field content will be used as a title of the application. See first line of `NOTICE.txt` file. |
+| copyright | string | Field content will be used as a copyright message. See second line of `NOTICE.txt` file. |
+| description | string | Field content will be used as notice file description. See third line of `NOTICE.txt` file. |
+| dependencies | array | If any dependency name mentioned, it will be automatically added even if it is not a first-level dependency. |
+| devDependencies | array | If any dependency name mentioned, it will be added when it is referenced in devDependency section. |
+| search | array | Pipeline will search for package.json/go.mod files mentioned here. Globstar format is supported ie. `x/**/go.mod`. |

noticegen/config.yaml

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+---
+
+title: "Mattermost Focalboard"
+copyright: "©2015-present Mattermost,Inc. All Rights Reserved. See LICENSE for license information."
+description: "This document includes a list of open source components used in Mattermost Focalboard, including those that have been modified."
+search:
+  - "mattermost-plugin/go.mod"
+  - "mattermost-plugin/build/go.mod"
+  - "server/go.mod"
+  - "linux/go.mod"
+dependencies: []
+devDependencies: []
+
+...