chore: gateway backend config initialisation improvements

[atzoum]

Description

• Respect the context in gateway.backendConfigSubscriber
• Wait until the first configuration has been consumed by the gateway before starting gateway's http listeners
• Add some more information in the Workspace Config changed log, such as the number of workspaces and total sources contained in the updated configuration.

Linear Ticket

Link

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.17% ⚠️

Comparison is base (2bc503d) 68.27% compared to head (35fa612) 68.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3688      +/-   ##
==========================================
- Coverage   68.27%   68.10%   -0.17%     
==========================================
  Files         319      319              
  Lines       50447    50456       +9     
==========================================
- Hits        34442    34365      -77     
- Misses      13786    13865      +79     
- Partials     2219     2226       +7     

Files Changed	Coverage Δ
backend-config/backend-config.go	84.12% <100.00%> (-0.07%)	⬇️
gateway/gateway.go	80.00% <100.00%> (+0.17%)	⬆️

... and 10 files with indirect coverage changes

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[achettyiitr]

Should we honor context as well here? Same for below as well?

	switch {
	case <-gateway.configInitialisedChan
		return
	case <-ctx:
		return
	}

[atzoum]

Not necessary since:

1. the context passed to HandleT#StartWebHandler is exactly the same as the one passed to HandleT#Setup and HandleT#backendConfigSubscriber
2. the channel will be closed if when the context is canceled, this happens inside HandleT#backendConfigSubscriber