chore: GitHub action for notification

[mj52951]

Added a GitHub action for notifying Sygma partners on Slack when files shared-config-mainnet.json or shared-config-test.json are modified. Also, for every PR that you want the Slack message to be sent, it will be needed to set the Restart Relayers label.

Currently it is not supported by the Slack API to use one webhook for sending messages to multiple channels. So, it will be needed to add both SLACK_WEBHOOK_URL1 and SLACK_WEBHOOK_URL2 repository variables for it to properly work for 2 channels. Accordingly, it will be needed to add more variables if the notification needs to be sent to more channels.

closes #95

[mj52951]

This is how the notification looks like on Slack

[mj52951]

This is how the updated notification looks like.

[mpetrun5]

Can we somehow trigger on-demand (tag, label...) ?
Most of the PRs don't actually require a relayer restart and are not changes that the partners should know about.

[mj52951]

Updated notification with separate checks for mainnet and testnet changes.