Allow customising the jinja2.Environment

[joaopgrassi]

Sometimes it might be desired to customize the Jinja Environment. For example, it might be useful to enable the features in Jinja to strip/trim blocks to make it easier to control whitespace rendering in templates.

This PR adds a possibility to pass multiple parameters (similar as the params that are fed into the template)which are passed into the jinja2.Environment as is. provide a new flag --trim-whitespace which enables both trim_blocks and lstrip_blocks in the Jinja environment

I came into this, while working on using the semconv code tool to generate the code files for the conventions in .NET: open-telemetry/opentelemetry-dotnet#2069

It proved very challenging to manage the whites paces/new lines and make the generated .cs file look right. In the end it got complex with the {%- option in jinja. I tried leaving it as is and using dotnet format but that also brought a myriad of other problems.

Now with strip/trim enabled, actual white space/new lines are rendered as is and it is MUCH easier to read and understand what the template is doing and what to expect of an output.

The usage of it would be:

docker run --rm \
  -v ${SCRIPT_DIR}/opentelemetry-specification/semantic_conventions/trace:/source \
  -v ${SCRIPT_DIR}/templates:/templates \
  -v ${ROOT_DIR}/Trace:/output \
  semconvgen \
  -f /source code \
  --template /templates/SemanticConventions.cs.j2 \
  --output /output/TraceSemanticConventions.cs \
  --trim-whitespace \ -> new flag
  -Dclass=TraceSemanticConventions \
  -DschemaUrl=$SCHEMA_URL \
  -Dpkg=OpenTelemetry.Trace

[Oberon00]

Sorry, I forgot to send my review!