[Code] Add a resolveSnippets API to resolve APM stacktrace item to source code

[mw-ding]

Summary

Addressing https://github.com/elastic/code/issues/1511 by providing a new GET /api/code/integration/resolveSnippets endpoint.

Example:
GET http://localhost:5601/api/code/integration/resolveSnippets?repoUri=github.com/Microsoft/TypeScript-Node-Starter&filePath=src/controllers/api.ts&lineNum=10

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/code

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[zfy0701]

the only thing I feel little strange is the naming

code/integration/resolveSnippets

the verb resolve here violate the rest style?

[mw-ding]

the only thing I feel little strange is the naming

code/integration/resolveSnippets

the verb resolve here violate the rest style?

spent quite some time on thinking about an appropriate API name as well, but failed to come up with anything satisfactory. let's just use code/integration/snippets and let it open to change in the future.

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request