chore: use trim_blocks and lstrip_blocks for jinja templates

[busunkim96]

Enable trim_blocks and lstrip_blocks in the Jinja environment to make it easier to manage whitespace and newlines. This PR may result in slight changes to the formatting of generated code.

https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.Environment
https://jinja.palletsprojects.com/en/2.11.x/templates/#whitespace-control

[codecov]

Codecov Report

Merging #838 (9d0d188) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #838   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1653   +45     
  Branches       328       337    +9     
=========================================
+ Hits          1608      1653   +45     

Impacted Files	Coverage Δ
gapic/schema/metadata.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
gapic/utils/checks.py	100.00% <100.00%> (ø)
gapic/utils/options.py	100.00% <100.00%> (ø)

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[software-dov]

How's the result?

[busunkim96]

@software-dov Still working out issues in a few places (hence the failing tests :)), but I think this should result in fewer instances of manual whitespace trimming {%-} and {-%}. https://jinja.palletsprojects.com/en/2.11.x/templates/#whitespace-control was the inspiration.

[software-dov]

More power to you for doing this. I remember having screaming fits when I first tried to make vaguely-python-whitespace templates.

[busunkim96]

Will get #774 over the line first and then merge this, since I suspect there will be merge conflicts otherwise.