[self hosting] Allow configuring the presigned URL

[mio-moto]

Hi, I'm having a not-so-terrible setup, where hosting services behind a compounding domain works mostly!

The museum service uses the s3.b2-eu-con.endpoint URL for presigning, which is slightly incompatible with portable reverse proxy configurations.

For example, a nginx configuration:

server {
  listen 80;
  ignore_invalid_headers off;
  
  location /minio/ {
    client_max_body_size 0;
    # omitted for brevity
    proxy_pass http://minio:3200/;
# [...]

with a museum.yaml:

s3:
  are_local_buckets: true
  b2-eu-cen:
    # omitted for brevity 
    endpoint: https://ente.domain.tld/minio
# [...]

creates a problem, that the pre-signed URLs are signed with https://ente.domain.tld/minio, the bucket however only gets a GET /b2-eu-cen/[...], therefore mismatching the pre-signing URLs.
As far as I'm aware, minio cannot be reconfigured to accept URL roots in such a way, that it would accept another root.

It would be nice to have a s3.b2-eu-cen.presign_url in place, so you would be able to presign the request with https://ente.domain.tld/, and reverse proxy like it's show above.

(Additionally it would save my system at least one roundtrip in a combined docker compose setup through the domain.)