Use URL-safe Base64 encoding for the topic name

[alexander-akhmetov]

Use url-safe base64 encoding to encode the topic name. It allows subscribing to topics with special characters in the name, for example $SYS/+, etc.

Fixes #111, #105, #90

How to test

1. Start Grafana with the plugin:

mage -v
yarn build
GRAFANA_VERSION=11.1.0 yarn run server

2. Run the test MQTT broker:

node scripts/test_broker.js

4. Create a new MQTT data source: tcp://host.docker.internal:1883

5. Subscribe to $SYS/my:topic/second/1 in Explore