Support additional HTTP content encodings (br, zstd)

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Compression significantly reduces the amount of bytes that need to be transferred, thus speeding up grafana. Right now, compression can be enabled with the `enable_gzip` configuration. `gzip` is a good baseline, but newer compression like `brotli` and especially `zstd` are more efficient. Both of these algorithms have support in browsers, with `zstd` having support in modern browser versions since it's rather new: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Encoding#browser_compatibility **What would you like to be added**: I would like to see a new configuration be added, that allows to specify more compression algorithms for content encoding: `content_encoding = zstd,br,deflate,gzip` In addition, we could think about marking the `enable_gzip` as deprecated. **Who is this feature for?** All users