[APM] refactoring sourcemap api to receive form-data

[cauemarcondes]

new post api to upload source map:

/api/apm/sourcemaps

curl --location --request POST 'http://localhost:5601/foo/api/apm/sourcemaps' \
--header 'kbn-xsrf: teste' \
--header 'Content-Type: multipart/form-data' \
--form 'service_name="foo"' \
--form 'service_version="1.0.0"' \
--form 'bundle_filepath="/test/e2e/general-usecase/bundle.js.map"' \
--form 'sourcemap="{\"version\":3,\"file\":\"static/js/main.chunk.js\",\"sources\":[\"fleet-source-map-client/src/index.css\",\"fleet-source-map-client/src/App.js\",\"webpack:///./src/index.css?bb0a\",\"fleet-source-map-client/src/index.js\",\"fleet-source-map-client/src/reportWebVitals.js\"],\"sourcesContent\":[\"content\"],\"mappings\":\"mapping\",\"sourceRoot\":\"\"}"'

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[cauemarcondes]

@axw is it ok to name the API /api/apm/sourcemaps?

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: f651662
• Storybooks Preview

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[axw]

LGTM!

@axw is it ok to name the API /api/apm/sourcemaps?

Yes that's fine, thanks.

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.x

This backport PR will be merged automatically after passing CI.