Add HTTP endpoint for TD validation

[farshidtz]

It would be nice to have an endpoint to validate TDs against the TD schema.

What should the operation/endpoint look like?

• POST /validation is not very RESTful.
• GET /validation is idempotent but some clients may not support a body in GET request.
• Add dry run query parameters to TD CRUD operation. E.g. POST /td?dryrun.

[dschowta]

Validation can be done in one of the following ways (motivated from this thread:

1. Provide a separate endpoint POST /dry-run-td so that a client can validate its TD. Here the POST method becomes idempotent which is not RESTFul

2. Use the same endpoint, but pass a flag to indicate it as a dry run. e.g.

   • POST /td?dry-run=true where the dry-run defaults to false when not given
   • POST /td?exec=false or POST /td?commit=false where the dry-run defaults to true when not given. This method complicates the existing endpoint

3. Do not perform dry run at all and leave it to the client. This has the problem that the client can not sync up always with the server about the schema unless the server exposes it.

4. Use GET /validation with body but some clients may not support a body in GET request.