[misc] user-agent tagging

[bkmartinjr]

On Python/R API-initiated HTTP requests, set User-Agent tags:

• R/Py environment
• Package version
• Means to add additional "application name" tag (allows discrimination of CI, WMG builder, etc)

This should propagate through:

• HTTP requests to CDN, eg, fetch of release.json, mirrors.json
• TileDB requests to S3
• Other requests, e.g., H5AD fetch via API which is currently using fsspec

To do:

• [x] Python API - TileDB requests. (#1193) ~- [ ] R API - TiledDB requests~
• [x] Python API - all other HTTP requests (#1193) ~- [ ] R API - all other HTTP requests~ ~- [ ] R unit tests set app flag~
• [x] Python unit tests set app flag (#1193)
• [x] Python download_source_h5ad (#1193) ~- [ ] R download dataset file (equivalent of download_source_h5ad)~
• [x] Builder fetch of datasets manifest needs to set header ~- [ ] CI and other pipelines set custom "internal user" header~

For hints on setting the TileDB header config: TileDB-Inc/TileDB#4400 For common User-Agent header examples, see here

[ivirshup]

We've merged #1193, which checks a few of the boxes here. I've edited the top comment to show this.

[pablo-gar]

Other non-Census usage is now tracked here

https://github.com/chanzuckerberg/single-cell-data-portal/issues/7298

[pablo-gar]

R work is now tracked here https://github.com/chanzuckerberg/cellxgene-census/issues/1249

[pablo-gar]

@ivirshup if this has been completed

Builder fetch of datasets manifest needs to set header

Then we can close this ticket