feat: Copy slowStartConfig for Gloo upstreams

[nickcaballero]

Now that Gloo supports Envoy's slow start mode (solo-io/gloo#7810), this PR ensures that slowStartConfig is copied to the generated upstreams.

[stefanprodan]

Can you also update the Gloo version in e2e tests to latest stable please? It's here https://github.com/fluxcd/flagger/blob/main/test/gloo/install.sh#L5

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.09 🎉

Comparison is base (317d53a) 54.42% compared to head (11f8927) 54.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1455      +/-   ##
==========================================
+ Coverage   54.42%   54.52%   +0.09%     
==========================================
  Files          84       84              
  Lines       10049    10143      +94     
==========================================
+ Hits         5469     5530      +61     
- Misses       3925     3956      +31     
- Partials      655      657       +2     

Impacted Files	Coverage Δ
pkg/router/gloo.go	68.64% <0.00%> (+0.51%)	⬆️

... and 14 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[nickcaballero]

@stefanprodan Regarding failed test run:

I created the DoubleValue type based on the fact that there was already a Duration type, both of which model the API reference from Gloo. However, when looking at the Gloo CRD, DoubleValue and Duration are expressed as number and string respectively. I was able to confirm that the existing types using Duration don't actually parse correctly either.

I'm going to update the double/duration types to the Go primitives instead for the SlowStartConfig, but I suspect the same change is necessary for the existing types to work (ConnectionConfig specifically, which makes use of Duration in various places).

[aryan9600]

@nickcaballero could you please squash and rebase on main? should be good to merge after that

[stefanprodan]

LGTM

[aryan9600]

lgtm! thanks @nickcaballero 🎖️