ratelimits: Old to new conversion tool for overrides formats

[beautifulentropy]

Write a new Go tool in the sre-tools repo which:

1. pre-processes the rate-limits YAML to make the comments actual YAML fields;
2. converts the rate limit data to the new format (including coalescing overrides into the current tiers, but not simplifying tiers); and
3. post-processes the result into have comments again (or keeps it structured, and we update how boulder parses this file)