feat: update getUsageLimits API and add service client generator script

[awschristou]

This change updates the bearer token based CodeWhisperer service client with the current planned API changes relating to subscription details. onShowSubscription in the agentic chat controller was updated to call getUsageLimits from the API.

To support this change:

• we used to use a script from the VS Code extension repo whenever we updated the service model. The script isn't used in that repo anymore, so I copied generateServiceClient.ts from an older version in the VS Code extension repo, and altered it to work in this repo.
• npm run generateClients -w server/aws-lsp-codewhisperer can now be used whenever we want to update bearer-token-service.json

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 6.45161% with 29 lines in your changes missing coverage. Please review.

Project coverage is 65.76%. Comparing base (13108fa) to head (41c1064).

Files with missing lines	Patch %	Lines
...nguage-server/agenticChat/agenticChatController.ts	0.00%	25 Missing ⚠️
...p-codewhisperer/src/shared/codeWhispererService.ts	33.33%	4 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           feature/subscription    #1931      +/-   ##
========================================================
- Coverage                 65.83%   65.76%   -0.08%     
========================================================
  Files                       239      239              
  Lines                     50525    50546      +21     
  Branches                   3362     3356       -6     
========================================================
- Hits                      33265    33242      -23     
- Misses                    17200    17244      +44     
  Partials                     60       60              

Flag	Coverage Δ
unittests	65.76% <6.45%> (-0.08%)	⬇️

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