Streamlining the release process for mattermost-plugins by integrating release and signing pipelines under the delivery platform.
This update automates signing and releasing by simply pushing a semver tag(e.g: v0.0.1) or by using the newly introduced Makefile targets.
```
make patch
make minor
make major
```
For Release Candidades(RC):
```
make patch-rc
make minor-rc
make major-rc
```
Signed-off-by: Akis Maziotis <akis.maziotis@mattermost.com>
