Add General purpose API wrapper util

[amishas157]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

This PR adds a general purpose API wrapper which could be used to call any API endpoint with given query params and headers.
There are two modules added:

• Client: Contains function to call an API. This supports retries in case there are throttling or service unavailable.
• Request: Contains function to build requestBody with given inputs such request type, headers etc

Also updated httptest library to support mocking http request when we want to see multiple results on subsequent api calls. This is particularly helpful when we want to test whether API retries work as expected or not.

Why

This will be used in stellar-etl to invoke various APIs-retool, github etc to fetch data and store in bigquery for analytics.

Known limitations

I did not add support for pagination as we don't have a use case as of now. However, this util can be extended in future as needed.

Questions

I am not sure if this is right place to add this library. I am open to relocating it where it finds better fit.