Add option to format summary as Markdown or HTML

[erichaagdev]

This would be more visually appealing when experiment results are viewed in a browser (e.g., in GitHub Actions summaries).

For example, instead of:

Performance Characteristics

Initial build time: 6m 28.381s
Build time with instant savings: 38.472s, 5m 49.909s savings
Build time with pending savings: 38.472s, 0.000s additional savings
Avoided cacheable tasks: 15 tasks, 2m 4.706s total saved execution time
Executed cacheable tasks: 3 tasks, 25.000s total execution time
Executed non-cacheable tasks: 148 tasks, 28.253s total execution time
Serialization factor: 1.86x first build, 1.97x second build

Not all cacheable tasks' outputs were taken from the build cache in the second build. This reduces the savings in task execution time.

See https://gradle.com/bvs/main/Gradle.md#performance-characteristics for details.

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Markdown could be used instead:

Performance Characteristics

Initial build time:	6m 28.381s
Build time with instant savings:	38.472s, 5m 49.909s savings
Build time with pending savings:	38.472s, 0.000s additional savings
Avoided cacheable tasks:	15 tasks, 2m 4.706s total saved execution time
Executed cacheable tasks:	3 tasks, 25.000s total execution time
Executed non-cacheable tasks:	148 tasks, 28.253s total execution time
Serialization factor:	1.86x first build, 1.97x second build

Warning

Not all cacheable tasks' outputs were taken from the build cache in the second build. This reduces the savings in task execution time.

See the Performance Characteristics section in the README for details.