Support custom summary.tmpl file

[andreasgerstmayr]

Support providing a custom summary.tmpl file.

@djaglowski is this what you had in mind for https://github.com/open-telemetry/opentelemetry-go-build-tools/pull/489#pullrequestreview-1866260923?

I didn't update any test input or output, so if everything is covered by unit tests, this PR shouldn't change the formatting of any existing changelog.

I noticed the lines here: https://github.com/open-telemetry/opentelemetry-go-build-tools/blob/ad9317af74bb648efb5b3f0c19b4697f0b468e65/chloggen/internal/chlog/summary.go#L57-L61 don't sort the changelog entries (sort.Strings should be used to sort them). I can update this PR to sort the entries by Component and Note if we want them sorted.

Resolves #488 Resolves #487

[djaglowski]

I can update this PR to sort the entries by Component and Note if we want them sorted.

Sorting would be great but let's keep it as a separate PR

[andreasgerstmayr]

I've dropped the external dependency now, and reverted some of the summary.tmpl changes, to keep it as simple as it was before.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ad9317a) 63.89% compared to head (1f9e485) 63.76%.

Files	Patch %	Lines
chloggen/internal/chlog/summary.go	90.32%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #501 +/- ## ========================================== - Coverage 63.89% 63.76% -0.14% ========================================== Files 39 39 Lines 2274 2274 ========================================== - Hits 1453 1450 -3 - Misses 677 679 +2 - Partials 144 145 +1 ```

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[djaglowski]

This looks good to me. We will need a changelog entry though :)

[andreasgerstmayr]

This looks good to me. We will need a changelog entry though :)

:smiley: Done. Thanks for the review!