OLIP - Opensearch implementation

Basic idea is cross app searching that OLIP will implement

* See https://docs.google.com/document/d/1bWLrUtpUtAuHTd_7phSZnH4b08Dn1bHAaB_K2Ge8_hI/edit#heading=h.14mpx6a8znb7

Its nasty ... XML, and incomplete (no example of what wanted back)

- [ ] Think about online / offline extension
- [ ] Specifying whether to search offline or online or both
- [ ] Informing in results whether data is downloaded
- See https://gitlab.com/bibliosansfrontieres/olip/olip-api/issues/19
- [ ] Add to descriptor - see https://gitlab.com/bibliosansfrontieres/olip/olip-deploy/issues/2
- [x] Build and return opensearch descriptor in response to query /opensearch
- [x] Build and return (REMOTE) opensearch results in response to URL in descriptor
- [ ] Build local text based searcher at least of metadata and return results.
- [ ] Asked for any kind of tool to build index and format of index used https://gitlab.com/bibliosansfrontieres/olip/olip-api/issues/22
- [ ] Think about IIAB implementation

Start with the linked design document and the existing /opensearch behavior described in the issue. Clarify the expected descriptor and result format, then determine the scope for offline metadata search, indexing, and online/offline status; completion depends on agreed requirements and implementation of the remaining checklist items.

Written by the indexing model from the issue text.