Add OpenApi 3.0 Yaml document

[vtnerd]

I've mostly converted the LWS API markdown file into a OpenApi 3.0 YAML spec. We can generate more visually appealing HTML spec files this way. Additionally, all of the fields now have hard constraints on integer size, etc., and can be more tightly validated by checker tools. These tools also provide (mostly decent) JSON examples, so it should lower the barrier to getting people understanding how the spec works.

The spec includes the "drafts" for #603 #647 #1228 such that YAML document is basically complete. This also includes an additional field for /get_subaddrs which I will PR after #647 gets merged.

Some additional constraints need to be placed on since_tx_id and since_tx_block_hash such that either both are included or neither is included. Wasn't certain how to do this.