### Description
Adds a `metadata diff` command to show comparisons between two different sets of Hasura metadata.
```
# Show changes between server metadata and the exported metadata file:
hasura metadata diff
# Show changes between server metadata and that in local_metadata.yaml:
hasura metadata diff local_metadata.yaml
# Show changes between metadata from metadata.yaml and metadata_old.yaml:
hasura metadata diff metadata.yaml metadata_old.yaml
```
Also adds a `--dry-run` flag to `metadata apply` command which will print the diff and exit rather than actually applying the metadata.
### Affected components
- CLI
- Docs
### Related Issues
Close#3126, Close#3127
### Solution and Design
- Added `metadata_diff.go` and `metadata_diff_test.go`
### Steps to test and verify
```
hasura metadata export
# Make changes to migrations/metadata.yaml
hasura metadata diff
```
### Limitations, known bugs & workarounds
This is just a general-purpose diff.
A more contextual diff with the understanding of metadata can be added once #3072 is merged.
