Weaver registry diff: template.set_file_name("schema-diff.yaml") in jinja is ignored when no output dir is provided

[lmolkova]

See open-telemetry/semantic-conventions#2307.

It has jinja template with

{{- template.set_file_name("schema-diff.yaml") }}
...

following the same pattern as we have in codegen.

If I run it without -o option:

weaver registry diff -r  ./current --baseline-registry ./baseline --diff-template ./path/to/templates --diff-format yaml

the result is printed to stdout.

If I add -o ./diff then the result is written to ./diff/schema-diff.yaml.

Suggestion:

• if no output folder was configured, but jinja attempted to set file name, we can default to current folder or fail
• Use the same output config between codegen templates and diff templates instead of inventing a new one - weaver registry generate does not have -o param and does something else. If we prefer -o, let's do it consistently for both options.
• There is no need to provide default stdout experience for people coming with custom templates and custom config - assume file output as a default for them, they can always cat the file if they need stdout.

Related to #760 and #761