feat: rest db service

[grantmacken]

issue #53
enables rest interaction with xqerl db collections and XDM items

Uses cowboy rest handlers in conjunction with xQuery main modules,
to allow CRUD ops on xqerl database collections and items.

HTTP request methods that create, list, retrieve, delete xqerl db collection items
POST collection URI with request body and ''content-type' header and optional Slug creates a resource
GET collection URI lists collection resources
GET resource URI with accept' header retrieves serialized XDM item
PUT resource URI with request body updates resource
DELETE resource URI deletes resource
DELETE collection URI deletes all resources in collection
HEAD resource URI, determine if resource available. If available will return content-length header
HEAD collection URI, determine if collection available.
OPTIONS collection URI, allowable methods for collection.

checks: Curl is used to to check correct responses to HTTP requests
https://github.com/grantmacken/xqerl/actions/runs/1603743140

[zadean]

I've merged the last 2 PRs so this one is in conflict in a few places. Might need a rebase.

I've tried out what is done so far, and must say, Very cool!! :-)

[grantmacken]

@zadean thanks and best wishes for the season.. I am away over Xmas new year period, will start looking at this again in a couple of weeks.

[zadean]

Great! Sounds good. Enjoy the time!

[grantmacken]

two issues, I don't think are there yet

1. A Head request should return content-length. I am not sure if this is achievable outside of the static handler context.
2. The collection path should be variadic [...] much like the static handler route allows. A the mo I only allow for a collection one level deep.

[grantmacken]

closing this as work on this on new branch
ref #58